added findmax(A,dim) function to array.jl. 

hi all,

this is my attempt at implementing my [original feature request for a version of findmax that operates along a given index of an array](https://github.com/JuliaLang/julia/issues/6716).

I added some tests to test/reducedim.jl, although not entirely sure that's where they belong and how this needs to be set up to work with the full test suite.