Offline sync controls

# Summery

Adds the ability to set offline syncing properties. Allows for setting weather syncing should be done over Wifi and/or cellular networks. The connectionType property (see below) is readonly, as it is automatically set by connection changes pushed to SPSession from the SystemConfiguration.framework (using SCNetworkReachability).

Also adds the ability to force libspotify in offline mode and setting the bitrate of offline synced tracks.

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The following method and properties has been added to SPSession:

    @property (nonatomic, readwrite) BOOL forceOfflineMode
    @property (nonatomic, readwrite) BOOL allowSyncOverWifi
    @property (nonatomic, readwrite) BOOL allowSyncOverMobile
    @property (nonatomic, readonly) sp_connection_type connectionType
    
    - (void)setPreferredOfflineBitrate:(sp_bitrate)bitrate allowResync:(BOOL)allowResync