Add a bootstrap script to run on prepublish (really local install)

* Tries to install the grunt-cli global if you don't have it
* Runs `grunt`

See #6
Don't prompt for Grunt install on Travis CI
Actually bail if you tell us not to install grunt-cli
Merge pull request #21 from Impromptu/better-install-script

Add a bootstrap script to run on prepublish (really local install)