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
The existing remove method returns the message instance that was just deleted so that the caller can update MWI counts via ARI so either decrement the read or unread messages accordingly. We probably need to return an array of message instances deleted so that the caller can do the same.

Or will you be deleting the MWI mailbox counts as part of calling this? Still sounds like it might be a good idea to add this in case the caller wants to do something like this.