Namespace based API and get rid of virtual inline functions for RtMidi(In|Out).

This patch includes more than it was intended to. The changes include:
• provide a namespace rtmidi to simplify class names
• Make all functions of Midi(In|Out)? non-virtual.
  Virtual inline functions have several pitfalls.
• provide Dummies for the new ALSA api.
• provide workaround for the major API breaks.
• use references for passing certain pointer arguments and deprecate
  the pointer versions of these functions.