Add support for ZMQ4
* update tests to handle/check zeromq4
* return empty string if $msg_size is zero in recv
* add router-req test to expose $msg_size/FFI::Raw problem

Closes #4. Thanks @klaus for the test and suggestions.
Refactor constants generation script
* use more readable variable names
* add missing generated output
* note that we use the 4-x repo to generate constants as well now

Regenerate constants to include everything through 4.0.3
add travis support for installing and testing zmq4