Add environment support based on git branch name.

[chrisbaldauf]

This adds a --environments command line parameter to the init command This creates a servers.json file that lists the servers for each environment. The current git branch name will be used to determine which servers to load and a branch that has no matching entry in the servers.json file will return an empty list (as in the case of a feature branch). See the readme for more detailed information

--environments will take precedence over --servers.

If not passed, a flat list of servers will be used.

[chrisbaldauf]

@maclennann @swivelhinges @potashj @ryanbreen

[ryanbreen]

LGTM!

[maclennann]

:shipit: