Consolidate search results and artist pages

Search results no longer separate results by spotify and local tracks.
Mopidy has support for several plugins and it no longer makes sense to
only support spotify and local backends. To that end, the local tracks
tab has been removed from search pages and artist pages. These results
are now displayed as just tracks.
Merge pull request #13 from connrs/feature-consolidate-search-results

Consolidate search results and artist pages
Search is now a little more discerning.

Until the v1.0 Mopidy Core changes arrive, jukePi will now only search
using providers specified in the config.searchUris array (defaulting to
spotify)

In addition, jukePi retrieves the available URI schemes at launch and
will identify searches that include a matching URI. If found, it will
perform a lookup instead of a search.

Once the new core features arrive, I'll allow you to specify which
backend library provider you wish to use.
Merge pull request #14 from connrs/feature-search-uris

Search is now a little more discerning.