Add convenience method for IMAP flavour of UTF-7

Mailbox names are 7-bit. Some mailbox names has to be converted to a
specialized form of UTF-7 that IMAP uses in order to fit the 7-bit
requirement. Core Foundation has support for converting to and from this
encoding. This is however not present in the Foundation layer, which
means that we have to invoke Core Foundation routines and make sure that
we work well with ARC.

Add a convenience method that converts NSString to the modified version
of UTF-7 using the functionality available in Core Foundation.

Signed-off-by: Marcus Karlsson <mk@acc.umu.se>