Merge pull request #236 from hintjens/master

Updated NEWS for release 2.0.1
Merge pull request #237 from hintjens/master

Fixed authentication classes to work over ZMQ v3.2
Merge pull request #238 from hintjens/master

Updated build instructions
Merge pull request #240 from hintjens/master

Updated library ABI version
Was not building properly without libsodium installed