Credit to dk379 and vzaliva for the changes, which include:
* handle single-part messages of type image
* refactor: remove duplicated code for handling of text parts
* consistent reply subjects - always prefixed by "MMS LJ:"
* Verizon gateway requires subject - so copy body there
* include reason code in case of posting errors (but omit in subject)
* handle duplicate image filenames, which some phones use
* deal with addresses enclosed in angle brackets