Track skipping, initial map marker, and launch screen

Added ability to skip between tracks with previous/next buttons. Previous button skips to beginning of current track if playback position is greater than 5 seconds. When current track finishes next track in playlist starts (playlists currently only work when track is started from album view). Fixes #87.

Added map marker over current location listing top tracks.

Added launch screen which waits until Rdio is loaded before showing main screen. Partially addresses #42.