Form Manager and Date Field support

This pull request contains the following enhancements to the Publisher:
1. Added a form management script which allows pieces of logic (plugins) to be specified on a per field basis
2. The add-asset page has been rewritten to use this form manager script to handle validations and form submissions
2. The form template provided to the add-asset and edit-asset partials now contain a isDate field which can be used to render date fields