[WIP] Port of DOP853

This is an almost complete port of DOP853 (the dense output functionality is still missing) with quite good performance (https://groups.google.com/forum/#!searchin/julia-users/dop853/julia-users/41B3qULjF-0/WqGa-Lc04AkJ), approximately 2-3x slower than Fortran (without compiler optimizations) for longer runs. Since the 5-th order version DOPRI5 looks rather similar implementing it as well should not be a huge amount of work

See also: #25

To Do:
* [ ] API adjustment
* [ ] Dense output
* [X] DOPRI5
* [X] Write tests
* [ ] Documentation

* What else?
* Is it a good idea to separate different solver families in different source files?
Maybe we could also have a look at `DifferentiableFunction` et al in [Optim.jl](https://github.com/JuliaOpt/Optim.jl)?