Add git tree library.

gitbot toolchain will be useful as a supporting toolkit for running
actions. One of the essential components is to interact with version
control systems to pull down and manipulate the source. The git tree
module will help in interfacing with git.

- Add a git module to gitbot library
- Add tests for git module
- Add ability to add and remove git tags
- Add a command builder class to facilitate interacting with
  git commands.