Fixes #199 - Canceling edits should revert change on edit Asset view

This fixes GH-199 by storing the original model data in a temporary model. On cancel we can get the old attributes from the temporary model.

I didn't add any unit tests. Not really sure how to unit test views too well. Any suggestions?

Also my version of coffeescript is 1.7.1. Looks like the current build uses 1.6.1.