Allow to set a custom working directory.

This is at least useful for testing, but it can be useful to preserve dependencies for deployment setups that destroy the current directory.
I'm not totally sold in the variable name, I only used that because it similar to Bundler's BUNDLER_APP_CONFIG.
Remove github restriction.

I know, I know ...
Add Mercurial support.

Fixes #6
Fix command type.
Merge pull request #18 from calavera/mercurial

Add support for Mercurial