SDE integrator fixes

I guess we should merge this now, even though in the long run we need a lot more extensive testing of the whole numerical integration machinery and also the `ExplicitStateUpdater` code needs some simplifying refactoring. But for now, the situation in this branch is certainly better than in master where the milstein integrator leads to incorrect results for multiple stochastic variables. Non-stochastic equations and stochastic equations with a single variable should not be affected by the changes in this branch. Closes #330 

Long tests here (the Cython on Python 3.3. did not finish in time, though...): https://travis-ci.org/brian-team/brian2/builds/38477925