Adds error constant

Adds error contants to libnanomsg.
Makes the test helper bind & connect functions to retun an endpoint
Adds [unstable] attribute to public functions
Adds a to_raw function in Protoco

Removes the 'match' in Socket::new.
Fixes issue #44.
Fixes typo in comments
Merge pull request #53 from blabaere/master

Fixes issues #44 & #45