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 
Agreed.

> On Oct 1, 2014, at 1:02 AM, Daniel Pupius <notifications@github.com> wrote:
> 
> In go/types.go:
> 
> > +	Mods                []Mod
> > +	ConditionalOperator string `json:"condition_operator"`
> > +	Conditions          []Condition
> > +}
> > +
> > +// FlagValue returns the value of a modified flag for the receiver.
> > +func (v *Variant) FlagValue(name string) interface{} {
> > +	for _, m := range v.Mods {
> > +		if m.FlagName == name {
> > +			return m.Value
> > +		}
> > +	}
> > +	return nil
> > +}
> > +
> > +const (
> I think that's fine in this case, but in general it's likely you end off using at several points through the file. So while it's not a big deal, I prefer grouping them. It also avoids splitting up a receiver methods.
> 
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