zeromq / cppzmq / 142

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travis-ci

add front and back (#269)

* 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

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