Added background listener thread & other enhancements

Background listener thread listens to server messages for status changes (to
keep Client in sync with server state changes). This was added as a member
to the ReceiverClient class, who manages creating/destroying the thread.

Fixed bug in which input changes were not being set up on fragment connection.

Fixed handling Volume status message interpretation if Server gives
non-numeric result. (un-documented server behavior)

Added more debug messages for eISCP framing error.

Added (VERY) preliminary 'logger' support.

Removed unused code from Eiscp class.