ZeroMQ Multipart message as deque

I originally designed my zmq::Socket class extending the zmq::socket_t class, as defined in the standard C++ wrapper included in the ØMQ 2.x distribution, to provide a more intuitive multipart message management, basing it on a std::vector container.

This is fine for building up multipart messages on receive, but it gets clumsy when we want to modify an existing multipart message, typically to put an envelope around it, or to strip that envelope out.

For this kind of manipulation I think is much better using std::deque.

The previous changes to the this class, till the latest introduction of a couple of methods to send and receive ints, were not dramatic ones. But this redesign breaks the original interface, a deque does not provide a reserve() method as the vector does, and I often used it in my code when creating a multipart message. Besides, I have also changed the blockingRecv() signature, since I found out that delegating to that method the check of the actual number of frames in a multipart message didn't add much value to the resulting code. On the contrary, it made it a bit less intuitive.

For this reason, I left on github the old include file, and I created a new one, zmq2a.h, where the deque version of multipart messages is defined and used.

Again, here is the change applied:

- zmq::Frames, type used for multipart messages, now is a typedef for a deque of strings.
- blockingRecv() now does not have any input parameter, it still throws an zmq::error_t exception, but only in case of failure receiving a frame in the message.

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