fix isdiscrete logic check in simulate (and save a few calls to getSampleTime).  turns out this was never working correctly for simulink models because of the previous transpose bug
fix atlas example, which was trying to query a DT trajectory at a non-knot point
also failed when T was not a multiple of dt
Merge pull request #400 from RussTedrake/default_simulink_solver

fix isdiscrete logic check in simulate (and save a few calls to getSampl...