If a play was not reported due to a network error, it is saved to the local database and retried when the next play is (successfully) reported.

Fixes #199.

Tested the following scenario's:

• Transitioning from one track to the next should create a play.
• Transitioning from one track to itself (on repeat) should create a play.
• Skipping to the next track should not create a play.
• Stopping playback should not create a play.
• Playback stopping due to the end of the playlist being reached naturally should create a play.