[transport/http] adding request and response encoders for Protobuf

[jprobinson]

Bringing these over from our marvin framework now that we can use Gizmo + go-kit in Google App Engine (hooray!)

I'm not 100% sure what we should do in the case of error while marshalling the protobuf so just commenting with a frowny face for now. Any thoughts for master...jprobinson:encode-proto#diff-05760750cf2e3aac59bd773896ee182fR202?

This resolves #808.

[jprobinson]

I've pushed updates with the new functions in a new subpackage: transport/http/proto

[jprobinson]

I've been thinking about the package name a bit and proto may be a bit annoying as it matches the main proto package. Any thoughts on naming it pb or httppb?

[peterbourgon]

Go kit consumers are already essentially required to make use of import aliases to get e.g. transport/http into their lexical scope. I think for now it's best to leverage that assumption, keep with the naming conventions, and stick with proto. (When we get around to a repo and package restructuring, I'll probably stick kit as a prefix to most package names.)

[peterbourgon]

Cool, I like this well enough. Any objections? I'll merge in a couple days otherwise.

[jprobinson]

I don't mean to be a bugger, but any chance we can merge this one soon-ish? I'm working on some internal docs/boilerplates and would love to include this lil tidbit.

[peterbourgon]

Oh! I thought I'd done it. Mea culpa.

[jprobinson]

Thanks, Peter!