Work around a bug in setting and getting the mtime in python 2

See http://bugs.python.org/issue12904.  Basically, we can get the mtime in
nanosecond precision, but only set it in microsecond precision.  This means
that the shutil.copy2 will not set the destination's mtime to exactly the same
mtime as our source.  The end result is that we can *always* end up copying
the extension because the source always appears newer.  We add a microsecond of fudge time when checking to see
if the source is newer than the destination to get around this.

This bug is fixed in Python 3.3+, I believe.

IPython/html/nbextensions.py

@@ -61,7 +61,9 b' def _should_copy(src, dest, verbose=1):'
     """should a file be copied?"""
     if not os.path.exists(dest):
         return True
-    if os.stat(dest).st_mtime < os.stat(src).st_mtime:
+    if os.stat(src).st_mtime - os.stat(dest).st_mtime > 1e-6:
+        # we add a fudge factor to work around a bug in python 2.x
+        # that was fixed in python 3.x: http://bugs.python.org/issue12904
         if verbose >= 2:
             print("%s is out of date" % dest)
         return True