Did not signal error when trying to send empty message

- zmsg_send () was sending zero frames, which was meaningless
- now returns -1
- added test case for this
Merge pull request #206 from hintjens/master

Problem: zmsg_send() does nothing when given an empty message
Fixed issue #208
Added comment on iteration
Merge pull request #209 from hintjens/master

Fixed issue #208
Added note about tivotisation
Merge pull request #210 from hintjens/master

Added note to clarify policy for embedded apps
Use zmq_strerror to show real error cause
Removed definition of 'inline' for Windows

- see issue #212
- it may be necessary to add this back for some MSVC versions
Merge pull request #211 from hintjens/master

Was not showing ETERM properly