CRM-15578 - Attachment API - Add support for CRUD'ing files

The File API is entirely inadequate to managing files. The Attachment API
provides a pseudo-entity which represents a record in civicrm_file combined
with a record in civicrm_entity_file as well as the underlying file content.

(See also: discussions on civicrm-api mailing list circa early Dec 2014.)
CRM-15578 - civicrm/ajax/attachment - Add endpoint for creating attachments using HTTP POST (multipart)
CRM_Core_Transaction - Doc fix
CRM-15578 - crmUiVisible - Add a helper for hiding elements while preserving layout

ngShow/ngHide use "display:none" to hide elements -- which modifies layout.
That's normally appropriate, but sometimes one wants to hide things without
changing layout ("visibility:hidden").
CRM-15578 - crmAttachment - Angular module for file attachments
CRM-15578 - crmMailing2 - Incorporate crmAttachments
REVERT - Testing tweaks - partials/crmMailing2/edit-unified2.html