Expose CMaildir to Lua

There's more I hope to do, but I think there's some useful functionality.  Let me know if you don't like any of it.
* Expose CMaildir to Lua code
* get_folders() now calls a "filter_maildirs" Lua function as an additional filter (as well as maildir_limit).  It's passed the CMaildir (wrapped to expose name, path, unread/total_messages, and a matches_regexp method) and returns a true/false value.
* Replace "CMaildir *" with "shared_ptr<CMaildir>" to simplify sharing objects with Lua.
* Also do the same with "CMessage *" (to be used)
* Add CMaildir::matches_regexp - like CMaildir::matches_filter, but doesn't handle "new"/"all" specially.