Features/229 db upgrade

This adds a versioning extension to FindMyDevice so we can track database schema changes.

You should be able to properly track version upgrades, downgrades and will abort if you've merged branches which contain changes to the schema which create multiple heads on the database version.

I've also added a Makefile to run a gospec testsuite against a database named findmydevice_test.  I'd like to get those tests merged in with @nchapman's travis tests.

Closes #229
I don't see the `sql/schema_changelog.txt` file in the repo or this PR, and this seems to be the only mention of it. Do we need to put a dummy file in the repo or a sample changelog entry?