#220: extract_messages fails on backslash-escaped quotes at the end of a string

If a translation string ends in an escaped quote (\") or apostrophe (\) the message extractors will throw an error.

Repro case:

1. Create a mako template containing this:

   ${_("\"")}
   ${_('\'')}
   

2. Set up babel and extract messages:

   $ python setup.py extract_messages
   Traceback (most recent call last):
     File "setup.py", line 67, in <module>
       """,
     File "/usr/lib/python2.7/distutils/core.py", line 152, in setup
       dist.run_commands()
     File "/usr/lib/python2.7/distutils/dist.py", line 953, in run_commands
       self.run_command(cmd)
     File "/usr/lib/python2.7/distutils/dist.py", line 972, in run_command
       cmd_obj.run()
     File "/home/nick/virtualenv/local/lib/python2.7/site-packages/babel/messages/frontend.py", line 314, in run
       for filename, lineno, message, comments in extracted:
     File "/home/nick/virtualenv/local/lib/python2.7/site-packages/babel/messages/extract.py", line 171, in extract_from_dir
       strip_comment_tags):
     File "/home/nick/virtualenv/local/lib/python2.7/site-packages/babel/messages/extract.py", line 201, in extract_from_file
       strip_comment_tags))
     File "/home/nick/virtualenv/local/lib/python2.7/site-packages/babel/messages/extract.py", line 285, in extract
       for lineno, funcname, messages, comments in results:
     File "/usr/lib/python2.7/dist-packages/mako/ext/babelplugin.py", line 29, in extract
       input_encoding=encoding).parse()
     File "/usr/lib/python2.7/dist-packages/mako/lexer.py", line 209, in parse
       if self.match_expression():
     File "/usr/lib/python2.7/dist-packages/mako/lexer.py", line 360, in match_expression
       text, end = self.parse_until_text(r'\|', r'}')
     File "/usr/lib/python2.7/dist-packages/mako/lexer.py", line 105, in parse_until_text
       **self.exception_kwargs)
   mako.exceptions.SyntaxException: Unmatched '"' at line: 1 char: 8