Remap exceptions to Unix_error where possible

Only exceptions that don't correspond to a standard Unix exception
are raised as ZMQ_exception.
Fix raise of ZMQ specific exceptions and add a unittest for EINTR. Also add a location to all exceptions using the __FUNCTION__ C macro
Test raising specific ZMQ exception
- Use zmq c library function names for unix error function name
- In case of errors, make sure do deallocate any zmq messages to aviod memory leak
- Use 'bracket' function for unittests to make sure context and sockets are released in case of failure
- Remove unused comments
Fix order on unix exceptions
Merge pull request #12 from issuu/remap_exn

Remap exceptions