Added zframe_sendmem + zero-copy version + tests

Currently you need to allocate a zframe_t on the heap and use zframe_send to send it. Additionally, this yields at least two SLOC.

This pull requests adds zframe_sendmem and zframe_sendmem_zero_copy.
These functions allow the user to send a ZMQ message part (=CZMQ frame) in a singl&e line (with or without MORE flag), without unneccessary heap allocations. Even if these functions do not use frames internally, I think the best place for them is zframe.c, because it looks like a wrapper for
```
zframe_t* frame = zframe_new();
zframe_send(&frame);
```
to the user.

Please feel free to comment!