Add packaging module for Gentoo Portage.

This is in no way comprehensive enough to cover all use cases, but hopefully is sufficient to cover the common ones.

The module uses:
- the "emerge" command (installed by default, of course) to perform actual work
- the "equery" command (installed by gentoolkit) to perform checking

I understand that there was a discussion in the mailing list some times ago (https://groups.google.com/forum/#!topic/ansible-project/1Q3CpQVc4kw), which mentions about using the python library as an alternative to "emerge" + "equery". I stick with the latter to mimic what a sys admin would actually do on the system.