Fix error in recurrence when length is 2

[heltonmc]

There is an error for the AbstractRange arguments when length(nu) ==2.

julia> besselk(3:4, 1.2)
2-element Vector{Float64}:
 3.910688631242223
 0.0

julia> besseli(3:4, 1.2)
2-element Vector{Float64}:
 0.0
 0.005800666221875795

This is a small fix and I also added some extra docs and tests to release v0.2.8.

I think we should also export all the other functions and release v0.3.0 next. We should ask to contribute the SpecialFunctions.jl port of loggamma and logabsgamma which will separate out native julia code which we will need here. This will also close the concerns in #27 (comment).