Initial implementation of ember-parallel.js

Initial implementation includes:

  - Ember.computed.promise: a computed property that returns the
    resolved value of a promise as the property value
  - Ember.computed.parallel.map: a computed property that maps over the
    given list inside web workers using parallel.js and returns the
    result
  - Ember.computed.parallel.reduce: a computed property that reduces
    over the given list inside web workers using parallel.js and returns
    the result
  - Ember.computed.parallel.spawn: a computed property that loads the
    given data into a web worker and returns the result
  - Em.Object.toJSON: a generic method that transforms Ember Objects
    into POJOs (which are required for web worker support)
  - Em.JSONify: a function that turns Ember objects into POJOs (using
    *.toJSON)

Also includes some grunt build stuff