Compile against gmime 2.6

  There is no actual usage of the library yet, and the build-deps
 are not updated.

  This updates issue #101.
  Attempt to parse CMessage with GMime.

  This is conditionally compiled at the moment to avoid suffering
 other people undue annoyance.

  The message is parsed, I think.  But we're not yet using the
 parsed object for anything.

  Suggest the next step is to update the CMessage::header() function
 to use the gmime-parsed m_message member.  If that shows good results
 then we can port over the attachment handling.

  NOTE: If you want to use/test this code please set -DGMIME=1 in
 the Makefile.

  Updates #101.
  CMessage::header() now uses GMime.

  This updates #101.

  TODO: We still don't decode things - HELP?!

  For example I see this:

From: =?UTF-8?B?UmFwaGHDq2wgUmlnbw==?= <notifications@github.com>
  RFC 2047 decoding of headers!!

  This updates #101.

  Subject:, From:, To:, etc are decoded.
  Updated comments.
  Updated text of the TODO to refer to the current state
 of the GMime port
Fix compilation when GMIME is not defined
  CMessage::get_body() uses GMime.

  I'm not 100% sure this is correct, but minimal testing shows
 it doesn't die and _something_ is displayed.
  CMessage::save_attachment() + CMessage::attachments()

  Both these functions are now converted to use GMime.

  The execution isn't beautiful, but we've now converted the
 whole of CMessage to use GMime.

  TODO: Update src/binding.cc to handle attachments correctly
 then we're 100% complete.

  This updates #101.
  Removed old TODO markers.

  Since they have been done.

  Updates #101.
  Call g_mime_shutdown() when we terminate.

  This is part of #101 and ensures that we're nice and clean.
  Show the version of GMime we're built with/against.

  This is part of #101.