[mythtv] MythTV on Raspberry Pi Raspbian Stretch

[Peter Bennett]

I have committed changes to the code to support the new Stretch version 
of Raspbian.

Stuart - please can you update ansible to reflect changes in build 
dependencies when building on stretch -
- use libcec4-dev instead of libcec-dev
- add libtool-bin  and libxcb-shm0-dev

Also note - IMPORTANT - after installing raspbian stretch run these
sudo apt update
sudo apt dist-upgrade
sudo rpi-update

Without the rpi-update in particular, MythTV does not work.

Run time dependencies - add libcec4, libtag1v5, python-future, 
python-requests, python-requests-cache

I committed to master and fixes/29. I am mulling whether to cherry-pick 
into fixes/0.28.

I have not yet tested the plugins with stretch. I am not a user of 
plugins so my testing is rather superficial.

Peter