Style improvements

Hey Matt,

I'm planning to reference this package a bit at Google I/O, so I figured it'd be a good idea to pitch in and tidy it up a bit. This round of commits is aimed at making http://godoc.org/github.com/mjibson/appstats nicer to read by unexporting the parts of the package that users won't care about. I've probably missed a few, but this seems like a good checkpoint.

Dave.