Install with homebrew

[dideler]

Create homebrew and linuxbrew formulae so that git-mob can be brew installed.

Makes it more accessible for those who aren't familiar with Node and don't have it (nor npm) installed.

See https://github.com/zmwangx/homebrew-npm-noob for easily creating a formula for an npm package.

[jplsek]

For anyone that requires brew, I'm currently maintaining an unofficial tap:

brew install jplsek/tap/git-mob