Add basher support.

Hi,

I would like to get your thoughts on this package manager I've been writing: https://github.com/basherpm/basher and a blog post here: http://juanibiapina.com/articles/2014-08-20-basher-a-package-manager-for-shell-scripts/

This change is what would be needed for bats to be installed with basher like this:

    $ basher install sstephenson/bats

It doesn't support versions yet (only master), but that might be a good addition if people think is interesting.

There is also this discussion with bpkg that seems to be dorment: https://github.com/bpkg/bpkg/issues/17

Any feedback is appreciated.