On artist search play popular songs.

trackset of popular songs created on the fly and inserted in spotify
widget play button.
json with artist's songs are ordered by popularity, hence no sorting
needed.
maximum of results shown now restricted by
typeaheadjs's param.
switched indentation to spaces instead of tabs.
Merge branch 'feature-play-artist' into development
task updated
"Popular Songs" string of artists updated.

changed from "Popular Songs" to "Top tracks for <artist_name>".
Merge branch 'development'.
build for v0.2.0