Pyflakes

- Convert pyflakes to use newer _ast infrastructure rather than compiler.
- Support for new syntax in 2.7 (including set literals, set comprehensions,
and dictionary comprehensions).
- Make sure class names don't get bound until after class definition.

- Fix reporting for certain SyntaxErrors which lack line number
information.
- Check for syntax errors more rigorously.
- Support checking names used with the class decorator syntax in versions
of Python which have it.
- Detect local variables which are bound but never used.
- Handle permission errors when trying to read source files.
- Handle problems with the encoding of source files.
- Support importing dotted names so as not to incorrectly report them as
redefined unused names.
- Support all forms of the with statement.
- Consider static ``__all__`` definitions and avoid reporting unused names
if the names are listed there.
- Fix incorrect checking of class names with respect to the names of their
bases in the class statement.
- Support the ``__path__`` global in ``__init__.py``.

- Display more informative SyntaxError messages.
- Don't hang flymake with unmatched triple quotes (only report a single
line of source for a multiline syntax error).
- Recognize ``__builtins__`` as a defined name.
- Improve pyflakes support for python versions 2.3-2.5
- Support for if-else expressions and with statements.
- Warn instead of error on non-existent file paths.
- Check for ``__future__`` imports after other statements.
- Add reporting for some types of import shadowing.
- Improve reporting of unbound locals