Feat(app): Dynamically use app path from config.json in gruntfile.js

Before this change it would be very hard to rename your app folder after the structure was generated
from the config file.

Is there any reason why this is a bad idea and the app folder was essentially hardcoded 
by the config file?

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<h4>Other considerations that should probably be implemented</h4>
* Subpaths (feature, controller, decorator, etc...) in config.json should be used dynamically too
* Consider changing <em>{{ feature }}</em> and <em>[service]</em> notations to be <em><%= %></em> 
so that they play nice with grunt. Not sure exactly how this would be done though.