settings: Update settings validator to support empty iterables.
stream backend: Add StreamBackend, fixes #151

Adds a basic streaming backend simply handles streaming audio and nothing else.
I.e. no metadata beyond the URI we where given. #270 still needs to be solved
for actual metadata to make sense in this backend.
core: Update BaseLibraryProvider to not require refresh, search or find_exact.

These methods may now return None, and the core code has been updated to filter
out missing SearchResults.
core: Update tests with cases for filtering out None results.
audio: Move supported URI checking to mopidy.audio.utils

In order to avoid gstreamer imports leaking into more of our code I'm moving
this to a new utils class in audio.