[WIP] adding output at specified points

I just added the `initstep`, `minstep` and `maxstep` keywords specified in the API.
Output at specified time-points is still wip, as for a `tspan` of `[0.:10.:50.]` I currently get output at

```
tout = [0.0 6.0e-7 2.351e-5 0.0005 0.009 10.013 19.981 30.016 39.983 49.997 50.0]
```
Why do you have `abs` in `eps(abs(...))`? According to the documentation `eps(x)` is the [difference between `x` and the next larger number](http://docs.julialang.org/en/latest/stdlib/base/?highlight=eps#Base.eps).