firebase-ios-sdk

Rewrite lint.sh as check_lint.py (#3161)

* Rewrite lint.sh as check_lint.py

This makes several beneficial changes:

  * Runs in parallel: lint all now takes 5.5 seconds, down from 25.
  * Supports linting specific files, passed on the command-line.
  * Defaults to linting files changed since master, pass --all to lint
    everything.
  * Infers header language based on related files.
  * Lays the groundwork for linting more than just C++.
  * Adds support for linting python.

* Fix lint errors

* Use check_lint.py in check.sh

* Remove lint.sh

* Python lint configuration

* Enable python linting in travis