Drafted some general API documentation
Merge pull request #28 from JuliaLang/in/api_doc

Drafted some general API documentation
Duck typing and consistent return types (Vector{typeof(y0)}) for all solvers; see #7. Tests adjusted accordingly.
Changed order of arguments to F,y0,tspan and use varargs for tspan.
Add keyword jacobian for ode4s*.
Changed [t0 t1 ts...] to hcat(t0, t1, ts...) to make it work with Julia 0.2.1.
Definitions with single argument tspan added.
Removed tspan-varags versions. Tests adjusted.