API Clean Up + Inquirer.js

Overall there are a few things that I'd like to address in this update:

- [x] Bump when.js to version 3.0 and bind the context to the whole chain
- [x] Clean up the API by dropping `url` convention for a `template` argument
- [x] Allow API to determine whether a url or a local path was passed as an argument, and act accordingly
- [x] #12 - Replace flatiron/prompt with Inquirer.js 
- [x] injects [String.js](http://stringjs.com) into your ejs templates

this is ready for review now /cc @jenius (special attention to be paid [here](https://github.com/carrot/sprout/pull/19/files#diff-d77067754ddef8eb0ee88a1968b47ea1R86))

- closes #18 
- closes #11 since we've decided to just override automatically