[WIP] Port of DOP853

This is 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
* [ ] DOPRI5

* What else?
* Is it a good idea to separate different solver families in different source files?