Assets plug-in version 2

This is a complete re-write of the Generator Assets plug-in. It includes all off the features from the first version as well as:

1. Significant refactoring for clarity and malleability;
2. The addition of a (nascent) Photoshop document object model; and
3. Default asset specifications, as described in #175.

Because this is such a large change, I'm submitting a for-review-only pull request now so that @joelrbrandt can start taking a look, with the caveat that I will almost certainly continue to push bug fixes to this branch. I'm sure we'll have a lot to talk about!

CC @isonno @timothynoel @timriot Give it a test if you have chance!