Use ember-cli plugins instead of custom scripts for determine revision and generate yuidoc

I've been working in a ember-cli plugin for generate yuidoc with a nice command (`ember yuidoc`) and I came with https://github.com/cibernox/ember-cli-yuidoc. Works like a charm. 
It also provides a generator for generate a `yuidoc.json` with some sensitive defaults.

In the process, I extracted also another addon that determines the revision of a project based in the version in `package.json` and the abbreviated SHA, since has usage on its own usages (by example in the Brocfile)

Basically, they are replacements for to custom scripts already present in ember: 

* https://github.com/cibernox/ember.js/blob/master/bin/generate_docs.js
* https://github.com/cibernox/ember.js/blob/master/lib/calculate-version.js (this one has been basically copied and tweaked)

I am already using them in ember-cpm. 

I'd love any kind of feedback, I am not very experienced with node and its conventions.