StompFrameTransport's canRead function retries system call on EINTR signal

At my place of employment, when certain tests were run with tests using stompest, it was found that the stompest test would sometimes fail because of an interrupted system call error in StompFrameTransport's canRead function.  

Python does not automatically retry system calls when EINTR is received but there is a current Python Enhancement Proposal (https://www.python.org/dev/peps/pep-0475) which describes how system calls should be retried when an interrupt error is retrieved as they can happen for many different reasons. This behaviour is yet to be included in python.

This pull request involves a fix and a corresponding unit test for this issue.  All existing tests in the stompest/src/core/stompest/tests folder were run and there were no failures.