Add dependency for mkdirp@0.5.0 [fixes #4]
Amend README to show usage with ember-cli [fixes #3]

I've added an example of how to get this plugin to
work on an existing ember-cli based application.
Implement upstream promises [closes #2]

The upstream broccoli-caching-writer now supports the return of
promises from the updateCache method.

Local has been cleaned and updated to use as such.