Make ZAP optional for PLAIN mechanism
Merge pull request #587 from hurtonm/master

Make ZAP optional for CURVE mechanism
Small cleanup in pipe.cpp
Merge pull request #588 from hurtonm/master

Small code cleanup
Added security curve test

Test is skipped if no libsodium.
Added warning if libsodium not configured
Merge branch 'master' of git://github.com/zeromq/libzmq