Standarize codebase

As discussed we should refactor gcovr to allow future improvements of it.

The first step to take is to standarize the code base so we have a base to start from. This is the first commit to accomplish this goal. 

With this commit the code base is now 99% PEP8 compliant. The remaining 1% is left to later refactoring.

This commit alter the style of the code base, but not the logic. No logic has being changed.

The code base was checked with PEP8, PyFlakes and mccabe.

Regards.