Rewrite using Asset Manager Framework

[johnbillion]

As demonstrated by the large number of bugs reported for this plugin, the approach of mimicking an attachment ID does not hold up beyond standard use. A more fundamental fix is needed.

This plugin needs to be rewritten from scratch using the Asset Manager Framework once it's stable.

Update: AMF-WordPress is now available as an alpha version: https://github.com/humanmade/amf-wordpress/