Added ind2sub(::AbstractArray, ::Integer)

As per request https://github.com/JuliaLang/julia/issues/294#issuecomment-65828607 
If we also add tests that the internal promotion works as we want it for `Int8` to `BigInt`, I'd be strongly in favor. Some sparse/regular structures might have indexes larger than `typemax(Int)`, and the fewer bugs anyone experimenting with such structures find, the better.