Added support for the tracks endpoint

Added the SearchTracks method to the client to allow searching and
filtering tracks.
Added the GetTracks, GetTrack, GetTrackComments and GetTrackFavoriters
to the SoundCloudClient class.
Refactored some methods and improved the performance of constructing the
query strings by using StringBuilder instead of string concatenation.
Added an extension method to convert DateTime objects to the string
format expected by the SoundCloud api.
Added extension method to add parameters to the StringBuilder class for
the object using the query.