Add parallelDependencies object

This sucks in functions declared inside parallelDependencies object
(within the Ember object) so that you can use them inside your
functions.

See http://bengillies.net/talks/ember-parallel/#/slides/17 for a basic
description.
Fix dependency on spawn; wrap parallel promises in RSVP
Only use the initValue on first get.

Use the previously fulfilled value of the promise when the promise
changes due to dependencies. This stops the value of the computed
property changing more than necessary.
Add config options; add initValue to map
Update README with new features
Update version to 0.1.0

New features include:

  - ability to define functions for reuse within parallel jobs via
    parallelDependencies
  - Em.computed.parallel.map now allows you to define an initValue
  - ability to set default configuration (including evalPath which is
    required for IE support)