Optimize pep8/pyflakes tests

Instead of havind two tests for them, make it a single one in a shell script.
The "make pep8" and "make pyflakes" rules were removed. Now you can use:
  * make check-source: To check for pep8/pyflakes on modified files
  * make check-source-all: To check for pep8/pyflakes on *all* files

Change-Id: I5860331406bedda8fb6127f1ef68b1babc53fa7a
Add a git pre-commit hook

This will check for source problems every time one is going to commit something
One can put it on hooks like:

  cp tools/git-hooks/* .git/hooks/

If using git-stoq plugin, it will do it automatically when entering shell.

Change-Id: Ica2fd30aa087aac95b6e7b43966bccc36787ebc3