Twig.js support

This pull request adds support for the Twig.js templating engine.

I tested it and it seems to work fine, however, I'm not sure how the twig.js dependency should be installed if ``--template=twig`` is actually used to generate a new sails project.

Is it intended for the generator to generate the example docs using the passed template engine rather than the .ejs files by default?

Kudos on the great work on Sails!