Add CodeFile with templating support for models/$name.js

Adds the generation of the model definition script file. [See here for more info](http://apidocs.strongloop.com/loopback-boot/v/2.0.0-beta1#adding-custom-methods-to-models).

/to @bajtos or @raymondfeng 
Can we use lodash templates instead of ejs? Yeoman uses loadash.