Flake8 now Hg friendly

I apologize in advance to the authors of pep8 and Pyflakes, as Flake8 is a horrible hack on the top of those two tools. But it's really what we needed at Mozilla Services to check that anything that lands into our Python repositories follow these simple rules:
- PEP 8 compliant - No unused imports - No undefined names

So Flake8 is a simple script that calls pep8 and pyflakes and merges the output, so you get all warnings grouped by files, instead of having to run both tools one after the other. I've stripped all options for now and you just call the script over a single Python file, over a directory or by using a *.py glob-style pattern.

I added a few features on the top of it:
- Python files that starts with this header are skipped: # flake8: noqa - Lines that contains a # NOQA comment at the end will be skipped

In the 0.4 version I have added a Mercurial hook you can use to automatically control your code when you call the push or the qrefresh command.

Here's how you configure it in your .hgrc once Flake8 is installed:

commit = python:flake8.hg_hook

qrefresh = python:flake8.hg_hook


strict = 0

If strict option is set to 1, any warning will block the commit. When strict is set to 0, warnings are just displayed in the standard output. I use it in non-strict mode, so I just get a display of the issues my code has.

Stuff I might add in the future:
- A cleaner merge of pep8 and PyFlakes options - EDIT: Added in 0.5 ~~Acyclomatic complexity metric so I can print out "Hey what's up with all those loops ? flat is better than nested!"~~

A more ambitious feature would be to detect similar code patterns and warn the developer that the function he's adding looks a lot like function Foo in module Boo. This implies using a tool like CloneDigger, but also keeping in a local, centralized database, some kind of index of every piece of code that gets committed. This is mandatory to speed up the comparison work and avoid introducing huge lags as the code base grows. Mmmm that would be a good GSOC topic !