[mythtv] Add cppmyth library. Needed for the MythTV pvr addon
[mythtv] Initial release 1.9.20

It is the successor of addon 'pvr.mythtv.cmyth' and it is intented to replace it with the release of MythTV 0.28.
Merge pull request #342 from janbar/release-v1.9.18

[mythtv] Initial release 1.9.20