[Feature Request] Add a Tag for the git branch.

[lanceofwhichwedream]

Ansible is frequently run from a git repo and for testing purposes, typically run from a git feature branch to work out any kinks. Including git branch information in the data reported to datadog would allow for more feature rich events and better alerting.

[lanceofwhichwedream]

I've followed the contribution guidelines and created a Pull Request that would add this to the datadog plugin.

It should be compatible with git >=1.8.1 and requires the subprocess built-in

#58