Line continuations now terminate after one blank line (#2108)

Previously, we had

In [1]: 1\
...:
...:
...:
...:

In other words, no amount of blank lines would terminate after a line
continuation, in contrast with regular Python:

>>> 1\
...
1

This made things really annoying when I typed \ instead of a newline--quite
easy to do since they are right next to each other on the keyboard.

Now, we have

In [1]: 1\
...:
Out[1]: 1

This also fixes another related behavioral difference between IPython.  If a
space follows a line continuation character, it should be a
SyntaxError("unexpected character after line continuation character"), even if
the line is otherwise continuable, according to regular Python (e.g., `1 \ `
or `(1 + \ `).  This now consistent between the two.

Closes  #2108

IPython/core/inputsplitter.py

@@ -369,7 +369,7 b' class InputSplitter(object):'
         self.code, self._is_complete = None, None
 
         # Honor termination lines properly
-        if source.rstrip().endswith('\\'):
+        if source.endswith('\\\n'):
             return False
 
         self._update_indent(lines)

IPython/core/tests/test_inputsplitter.py

@@ -351,6 +351,22 b' class InputSplitterTestCase(unittest.TestCase):'
         self.isp.push(u'\xc3\xa9')
         self.isp.push(u"u'\xc3\xa9'")
 
+    def test_line_continuation(self):
+        """ Test issue #2108."""
+        isp = self.isp
+        # A blank line after a line continuation should not accept more
+        isp.push("1 \\\n\n")
+        self.assertFalse(isp.push_accepts_more())
+        # Whitespace after a \ is a SyntaxError.  The only way to test that
+        # here is to test that push doesn't accept more (as with
+        # test_syntax_error() above).
+        isp.push(r"1 \ ")
+        self.assertFalse(isp.push_accepts_more())
+        # Even if the line is continuable (c.f. the regular Python
+        # interpreter)
+        isp.push(r"(1 \ ")
+        self.assertFalse(isp.push_accepts_more())
+
 class InteractiveLoopTestCase(unittest.TestCase):
     """Tests for an interactive loop like a python shell.
     """