When the Python Interpreter exits, it first attempts to join any
outstanding threads.  This is problematic if a server is created
as a top-level variable or referenced by a reference cycle, as join()
will hang.  This adds cleanup behavior to the server thread's join().

Notable Changes:
  -Convert all str types to byte types at cython layer (ascii encoding)
  -Use six for packages that have different names in Python2/Python3
  -By default, unit tests are compiled/run in Python2.7 and Python3.4
  -Ensure MACOSX_BUILD_TARGET is at least 10.7