Add BaseMiddleware to vumi.middleware API for 3rd-party middleware that wants to support both transports and applications.
Add BaseMiddleware to vumi.middleware API for 3rd-party middleware that wants to support both transports and applications.

vishwaprakashmishra/xmatrix,TouK/vumi,harrissoerja/vumi,harrissoerja/vumi,TouK/vumi,vishwaprakashmishra/xmatrix,harrissoerja/vumi,TouK/vumi,vishwaprakashmishra/xmatrix