Need to add section on how/where to create tests in DevelopersGuide.md

[KrisThielemans]

As tests are an essential part, it needs to be documented how to add them. Structure could be

1. Overview of testing strategy
   CMake, Travis
2. Adding new tests
   a. C++
   b. Python
   c. MATLAB
3. Brief info on Travis

I think some of the generic text can just be copied from the SIRF paper. In any case, keep this brief!