Fixing issue preventing the correct read of images by full_html and reveal exporters.

Currently to see and images in the notebook inside a md cell you can use the prefix files/ to read the content of the directory where the ipynb lives. But, if you do a conversion using full_html or reveal exporters, the are not rendered ok because the previous version of this function erased `/files/` instead of `files/`. With this fix, the html files produced by full_html and reveal exporters can read the images in a correct way.

IPython/nbconvert/filters/strings.py

@@ -1,113 +1,113
 """String filters.
 
 Contains a collection of useful string manipulation filters for use in Jinja
 templates.
 """
 #-----------------------------------------------------------------------------
 # Copyright (c) 2013, the IPython Development Team.
 #
 # Distributed under the terms of the Modified BSD License.
 #
 # The full license is in the file COPYING.txt, distributed with this software.
 #-----------------------------------------------------------------------------
 
 #-----------------------------------------------------------------------------
 # Imports
 #-----------------------------------------------------------------------------
 
 # Our own imports
 import textwrap
 
 #-----------------------------------------------------------------------------
 # Functions
 #-----------------------------------------------------------------------------
 
 __all__ = [
     'wrap',
     'strip_dollars',
     'rm_fake',
     'python_comment',
     'get_lines'
 ]
 
 
 def wrap(text, width=100):
     """ 
     Intelligently wrap text.
     Wrap text without breaking words if possible.
     
     Parameters
     ----------
     text : str
         Text to wrap.
     width : int, optional
         Number of characters to wrap to, default 100.
     """
 
     split_text = text.split('\n')
     wrp = map(lambda x:textwrap.wrap(x,width), split_text)
     wrpd = map('\n'.join, wrp)
     return '\n'.join(wrpd)
 
 
 def strip_dollars(text):
     """
     Remove all dollar symbols from text
     
     Parameters
     ----------
     text : str
         Text to remove dollars from
     """
 
     return text.strip('$')
 
 
 def rm_fake(text):
     """
-    Remove all occurrences of '/files/' from text
+    Remove all occurrences of 'files/' from text
     
     Parameters
     ----------
     text : str
-        Text to remove '/files/' from
+        Text to remove 'files/' from
     """
-    return text.replace('/files/', '')
+    return text.replace('files/', '')
 
 
 def python_comment(text):
     """
     Build a Python comment line from input text.
     
     Parameters
     ----------
     text : str
         Text to comment out.
     """
     
     #Replace line breaks with line breaks and comment symbols.
     #Also add a comment symbol at the beginning to comment out
     #the first line.
     return '# '+'\n# '.join(text.split('\n')) 
 
 
 def get_lines(text, start=None,end=None):
     """
     Split the input text into separate lines and then return the 
     lines that the caller is interested in.
     
     Parameters
     ----------
     text : str
         Text to parse lines from.
     start : int, optional
         First line to grab from.
     end : int, optional
         Last line to grab from.
     """
     
     # Split the input into lines.
     lines = text.split("\n")
     
     # Return the right lines.
     return "\n".join(lines[start:end]) #re-join