Add support for valid human readable formats from SiteAttachment.
New base abstract attachment upload class.

This can be used across all our attachment admins.
Add a site level abstract attachment downloader.

This can be reused for all of our sites that use SiteAttachment.
Bump swat dependency as well.

This is required for SwatFileEntry features that were recently added and are being used in the new Attachment upload base class.
Add license
Add a getResolvedObjectClass() and use it.
Mark xml strings as translatable
Set the form's upload_status_server in build and add a method to get it.

This will allow for easier subclassing.
Merge pull request #64 from isaacgrant/attachment-upload

Attachment upload and download.