Give option on how git tracks submodules.

Allows user to decide if git submodule should track branches/tags or track commit hashes defined in the superproject.

Add track_branches parameter to the git module.

Defaults to track branches behavior.
Chnge option name to track_submodule_branches.

Update documentation to reflect what the actual effect of the option.
Merge pull request #55 from bibliotechy/add-track-submodule-branches-parameter

Add track submodule branches parameter