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.
This block may be better expressed (and free up lines 156-161) via:

```js
default:  function() {
  return this.props.themesDir || 'web'/wp-content/themes';
}.bind(this)
```