Add DAL functions in preparation for voicemail admin tool

context: all() - retrieves all contexts
folder: get(name) - retrieves a folder by name
folder: getNameOrDTMF(name, dtmf) - retrieves all folders matching either the name or the dtmf
mailbox: allContext(context) - retrieves all mailboxes matching the context
mailbox: countContext(context) - counts all mailboxes matching the context
message: removeMailbox(mailbox) - removes all messages belonging to a specific mailbox
This needs  a comment to explain why we are passing undefined to the create method.