Change CTCoreMessage.render and CTMIME.render to return NSData, not NSString,
and change CTSMTP.setData to take NSData to match.

This avoids interpreting the bytes that mailmime_write_mem gives us
as UTF-8, only to write them back out as UTF-8 when sending the
message.

As part of this, mention that the render methods return something
that needs to be released using mmap_string_unref.  This avoids
a small leak each time this method is called, because the render
call was using mmap_string_ref and then passing the bytes to
NSString.initWithBytes.  This was leaking the hashtable entry that
mmap_string_ref makes.
Take the old SenTestingKit-based tests and convert them to XCTest
(apart from the CTConnectedTest ones, which need a server configured
in the right way, and CTCoreAttachmentTests which need the types.plist
which I don't like anyway).

Fix the rest to match recent API changes.