zmqpp::socket::receive_raw also has int length

As with my previous pull request for send_raw, the buffer for receive_raw has an
int for length rather than a size_t. I added unit tests for testing sending >2GB
messages using zmqpp::socket and now something fails inside ZMQ. It returns with
EAGAIN.
Merge branch 'develop' of https://github.com/cmdevries/zmqpp into develop

Conflicts:
	src/tests/test_socket.cpp