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 pull request #53 from cmdevries/develop

zmqpp::socket::receive_raw also has int length