Merge pull request #371 from sappo/master

 put/get 8,16,32,64 bit numbers, and string in network byte order to a single frame
Small improvements to zframe_put/get integers

- return -1 on error, not 1
- unrolled loops are simpler and faster
- slightly tweaked API methods
- modified methods so error handling is last
- don't assert on invalid frames (frames are external data)
Merge pull request #372 from hintjens/master

Small improvements to zframe_put/get integers
updated documentation for zframe put/get api