Solution: Implement for socket_t, context_t, message_t and poller_t
Additionally remove dependency on <functional> by refactoring poller_t
and remove unused <unordered_map> include.
* add front and back
i'll get the last message in some situations，but have to do like this :
```
multimsg msgs;
auto msg = msgs[msgs.size() - 1];
func(msg.size());
auto *msg = msgs.end() - 1;
```
but, std::queue have some method 'front' and 'back'. so maybe i can simply do this ？ :
```
multimsg msgs;
auto msg = msgs.back();
```
* add test for front and back
For header only library like cppzmq, whitespace style inherited from
libzmq is too restrictive.
Solution: relaxing whitespace before parens from always to in control
statements only, increased max column width from 80 to 85 and removing
requirement of whitespace after template keyword.
Add comments to all functions. Optimize move constructor and move assigment operator, remove size check inside pop/remove/peek. Static assert in pushtyp/addtyp/poptyp in order to avoid pitfall with std::string (pushstr/addstr/popstr should be used for that). Add sanity check for poptyp, size of message must match desired type.
This class handles multipart messaging. It is the C++ equivalent of zmsg.h, which is part of CZMQ (the high-level C binding). Furthermore, it is a major improvement compared to zmsg.hpp, which is part of the examples in the ØMQ Guide. Unnecessary copying is avoided by using move semantics to efficiently add/remove parts.
