Merge pull request #814 from hurtonm/master

Replace assert with zmq_assert
cmake: fixed error 'install TARGETS given no ARCHIVE DESTINATION for static library target'
Merge pull request #815 from snikulov/cmake_install_update

cmake: fixed error 'install TARGETS given no ARCHIVE DESTINATION'
windows: changed tcp port from glrpc assigned to unassigned one
Merge pull request #816 from snikulov/fix_test_windows

windows: changed tcp port from glrpc assigned to unassigned one
Only export zmq_* symbols
Fix timestamp counter on s390/s390x

Fixes #811
Merge pull request #818 from Debian/s390x

Fix timestamp counter on s390/s390x
Merge pull request #817 from Debian/symbols

Only export zmq_* symbols
Implement socket_base_t::get_credential member function

The get_credential () member function returns
credential for the last peer we received message for.
The idea is that this function is used to implement user-level API.