Drafted some general API documentation

I think the time has now come to try to agree on some sort of document that describes the basic elements of a ODE solver interface in Julia.

The long discussions in the last few issues, has been very enlightening, but very hard to follow by outsiders who just wonder what the result of the discussion was.

I think this should be merged fairly soon, and then we can discuss details in later PR/issues. I have definitely ignored a few arguments, so please put them back in if you feel strongly for them.