go: fix golint errors and some general cleanup/consolidation.

+ Added comments for exported funcs and fixed some other nits per complaints from golint.
+ Consolidated shorter type definitions into a single file.

R: @tessr @dpup 
Because it's in violation of golint's rules.

A

> On Sep 30, 2014, at 5:53 PM, Tess Rinearson <notifications@github.com> wrote:
> 
> flagToVariantIDMap