Fixed issue where peek and pop on send_queue could act on different
entries.

After sending the sent entry is now not removed from the queue by a
pop but an explicit remove instead. Removal maintains the queue's
heap invariant.
Some small refactorings

- _nack_lines, _nack_lock, _nacks_before_resend => _sent_lines, _sent_lock, _sent_before_resend
- some reordering in the message processing
- referal to MESSAGE and COMMAND instances via self.__class__ instead of RepRapProtocol => allows sub classes to override MESSAGE and COMMAND definitions
RepetierTextualProtocol now makes use of overriding RepRapProtocol.MESSAGE_TEMPERATURE
Virtual printer did not send repetier-style target temperature messages while heating up
Version resource was defined twice