Genesis agnostic

Makes Evolution agnostic of Genesis Skeleton - WordPress

### Changes

* Prompt for themes directory instead of assuming default directory organization
* Template with `gruntfile` instead of copying it
* Create `WP_ENV` constant if it's not already defined
* Update tests

### Review

@ericclemmons
@germanny 
@ericrasch 
@jameswlane 

### Notes

After 0.3.0 goes live this project will most likely be rebranded to break away from Genesis Skeleton.
Yes. Yeoman uses the same templating engine as Grunt. Since this is now a template and not a copied file, we had to add the second `%` to keep Yeoman from trying to expand the variables. This variable is the only one we want it to expand (based off of the users answer in the Themes Dir prompt).