Snapcraft configuration for build (plus some helpers)

[ted-gould]

This branch creates a snapcraft configuration to build a snap of xTeVe. I packaged this so that I could run Ubuntu Core and my Raspberry Pi 3 and get everything auto-updating and secure.

Most of this diff is involved in the snapcraft configuration and helpers. I had to add a couple features to xTeVe to make it work better:

• Added a way to query the version so it automatically showed up in the package manifest
• Added a way to get the current tuner usage in the API so snapd won't upgrade it while someone is watching a stream
• Created two small helper utilities to make all that easier
• Added a param to disable updates as the snap is on a read-only file system and that doesn't make sense

I've uploaded the snap to the snap store but left it as unlisted. Happy to transfer the name over to the project.