Add hooks module, containing utility functions common to hook scripts.

This is a new 'hooks' module in RBTools. rbtools/hooks/common.py contains
functions common to all hook scripts, and rbtools/hooks/git.py contains
functions for Git hook scripts.

Testing Done:
- Tested the functions individually, and got the expected results.
- More testing was done in https://reviews.reviewboard.org/r/5403/ for the Git
  post-receive and pre-receive hook scripts that use this new module.

Reviewed at https://reviews.reviewboard.org/r/5543/