Add asyncio to the storage backends and most of the codebase. A lot of
it merely uses asyncio APIs, but still doesn't actually run several
things concurrently internally. Further improvements will be added on
top of these changes
Thanks to Thomas Grainger (@graingert) for a few useful pointers
related to asyncio.
* Revert "Don't test development Radicale"
This reverts commit 7a5241101e.
* Fix Radicale test setup
* Radicale is very tolerant
* Simplify test such that output is more predictable
* Runtime version check for Radicale
* Don't create user explicitly
* stylefix
* Shorter tracebacks
Travis logs are very hard to read
