Split up build tool resources and added gulp support

Adds support for Gulp to the generator

### Changes

* Split up build tool resources in /root dir
* Added gulpfile
* Added package.json specific to Gulp dependencies
* Added prompt to allow user to choose between Gulp and Grunt (defaulting to Grunt)

### Testing

* Clone the repo and switch to the `add-gulp-support` branch
* Run `npm link` to allow Yeoman to run this version vs the installed version
* Create a test WP project, run `yo evolved` from the root.
* Follow the instructions per usual, except choose Gulp when prompted
* Run `gulp:dev` to create dev assets
* Run `gulp build` to create dist assets
* Run `gulp watch` to create a watcher with LiveReload
* When you're done testing you'll want to remove the local symlink, run `npm r generator-evolved -g`

### Review

@germanny 
@ericrasch 
@jameswlane 
@weeirishman 
@ericclemmons

### Notes

* I have a Gulp PR for Evolved-Theme already open, but neither of these relies on the other since there's no changes to the themes themselves.
* I prefixed non-required variables with an underscore to make it easier to discern one from the other. Open to a discussion if this is a good idea or not.