Code style validation for Python using YAPF

[robertodr]

This PR enables the use of YAPF for Python code style validation. Have a look here for an example.

Some questions:

1. The format for passing lines to reformat is slightly different between clang-format and yapf:
   • The former can be invoked with clang-format -lines=start:end
   • The latter can be invoked with yapf --lines=start-end

I have inserted a check on the validator name in resolve_changes to get the correct argument set up for the actual call to the validator. This is not really clean, @nikolaykasyanov do you have suggestions on how to possibly improve on this?
2. I have put a test "Accepts a validator other than clang-format". Is that enough?

[nikolaykasyanov]

I wonder what does yapf --version print. If it mentions its name somehow we may use the output we get from validator to determine the tool. What do you think?

[robertodr]

Indeed, they print their name and version:

• yapf --version returns yapf 0.11.0
• clang-format --version returns clang-format version 3.9.1 (tags/RELEASE_391/final)

Maybe this could even go one step further and use validator --help to get both the tool name and the options to specify lines. I'll see what I can do.

[robertodr]

Coming back to this. I actually think this is the implementation that is least prone to breaking if the CLI of the validators changes. Thus, IMO, this is ready to go as-is 😉

[nikolaykasyanov]

@robertodr I see. Could you please fix latest commit's authorship? It looks like there's my email somehow.

[nikolaykasyanov]

Weird, it looks like a part of my latest commit is included in your commit. Maybe rebasing helps.

[robertodr]

Rebasing didn't seem to help...

[nikolaykasyanov]

It did help to get rid of unnecessary changes in diff though. Could you try using git comment amend --reset-author?

[robertodr]

Not quite sure that helped either... If you prefer, I can close and re-open this PR. It actually did help 😸 Mac OS X tests are however failing 😕

[nikolaykasyanov]

Ugh, seems to be Travis issue. Merging. Thank you! 🎉