Adds option only_stats for low memory scenarios.

Over at OpenCL.jl we repeatedly run into the problem that our tests failed on Travis due to out-of-memory errors. We mitigated this by starting a Julia instance for each test, but this is ugly and wastes time. So we determined that the core problem is that FactCheck retains references to the code that is run and that blocks garbage collection on the OpenCL side. On some OpenCL implementation all OpenCl objects have to be reference free before any can be collected... 

This PR adds a flag that can be set with FactCheck.onlystats(true), which prevents storing results in the allresults global array and instead directly stores the statistics.

For reference
JuliaGPU/OpenCL.jl#61
JuliaGPU/OpenCL.jl#43

cc @jakebolewski