Some numeric_limits and numeric helper functions were missing

[siposcsaba89]

Extended std::numeric_limits class with some functions needed to use it with yaml-cpp.

[siposcsaba89]

I was hoping for this! I have to jump through hoops to get inf.

A few comments. Also, you should probably add tests in https://github.com/nholthaus/units/blob/master/unitTests/main.cpp

Also added some unit tests for limits. Can you check them? Maybe you have some other tests in mind.