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

I've been working with my own protocol lately. Therefore I'd copied the zre_msg PUT and GET logic to add numbers and string to a single frame. As this seems like something lot of people might find useful, I introduced it to the zframe api.

The API maybe needs some tweaking to be easier to understand and use.

//sappo