RFC: turbojpeg imread/imwrite support

This needs some work, especially for generality, but can round-trip properly:
```
julia> import ImageView

julia> module x
       include("JPEG.jl")
       img = imread("/tmp/tonys_21.jpg")
       imwrite(img, "foo.jpg")
       img2 = imread("foo.jpg")
       end

julia> ImageView.display(x.img2)
```