Api 179 add tracking headers

One of the annoying things about Go is that since programs are compiled into standalone binaries, they won't include any files/assets/etc. So if I made a program that imported clever-go and called GetVersion(), it would always return "unknown" because the VERSION file would not be there to read.

The solution we've used so far is to make the version a constant and put it in a .go file like version.go. To keep it in sync with the VERSION file, we generate version.go via the Makefile. See https://github.com/Clever/whackanop/blob/master/Makefile#L25-L28 for an example. (sidenote: in whackanop and other binaries we gitignore version.go to make it explicit that it's not to be edited. In the case of a library like clever-go we cannot do that, since import needs to pull down the version.go file from git)