WIP: Test framework

I've started working on a small test framework for the ODE solvers. It uses FactCheck for verification, but I've made some modifications there as well, as well as used some code that is likely to move soon (JuliaLang/julia#2422), so this really isn't ready for merging yet.

But I very much like to hear what you think about the test API and the general look of the code so far =)