Add way to set accuracy in SLSQPFitter

Exactly what it says - adds an argument to `SLSQPFitter.__call__` that sets the `acc` parameter of `scipy.optimize.fmin_slsqp`.

I'm following the idiom already established in `__call__` for this, but it seems like it might make more sense if it were an attribute of the fitter object instead of being in `__call__`.  But that would be different from e.g. `maxiter` and `epsilon`.  Is this intended behavior, or do `maxiter` and `epsilon` have special status?  

cc @embray @nden