Adding install to Makefile

@lpw25 please check this carefully. I have little idea what I'm going. I am adding a "install" to the Makefile so I can add this to opam-repo-dev and the opam file can say:

 build: [
 [make "all"]
 [make "install" "BINDIR=%{bin}%"]

I'll not sure this is the best way to go about it, I've just been copying the style from lpw25/bin-doc