Currently cppzmq as relatively simple and header only library depends on rather
complex unit test framework googletest.
Current issues:
- Googletest requires downloading and building it every time on travis
as cache support is limited there
- Googletest build is signifficant with comparison to cppzmq unittests
total runtime
Solution: Port existing tests to Catch - header only C++ framework and
gain ~20% build speed up on travis.
Why Catch?
It is well know C++ header only testing framework. It works well, it is
being kept up to date and maintainers seem to pay attention to
community's comments and issues.
We can not use Catch2 currently as we still support pre-C++11 compilers.
* 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.
