Cors

Add proper CORS support directly in Primus instead of assuming that our transformers implement it correctly. Fixes #142. In addition to fixing this feature request in Primus it should also correctly fix the following bugs in the supported real-time "transformers":

LearnBoost/socket.io#12
LearnBoost/socket.io#764
LearnBoost/socket.io#194
LearnBoost/socket.io#1025
LearnBoost/socket.io#1442
LearnBoost/socket.io#547
LearnBoost/socket.io#1214
LearnBoost/socket.io#1046
LearnBoost/socket.io#986