Fix compilation issue against libzmq master.

Since libzmq master now aims toward 4.2 some issues with preprocessor #if
arised. This commit corrects those.
Add support for new ZMQ_POLLPRI flag.

I recently added a ZMQ_POLLPRI flags for polling events. This
adds the support of this flags to zmqpp.
Merge pull request #75 from xaqq/pollpri

POLLPRI support + compilation fixes