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
WIP - Add crmAttachment
WIP - Incorporate crmAttachments into crmMailing2