Example of supporting dep and modules

[cep21]

An example of what it would look like to support dep and modules at the same time.

[cep21]

This is mentioned as an alternative under https://github.com/golang/go/wiki/Modules#releasing-modules-v2-or-higher "Major subdirectory" approach. This is understandable by older versions of go, including dep. Go modules are also aware of this approach and will look in the /v3 directory for the right files.

On #53 you had a concern

only way that would work is if I copied resp and its sub-package into the main one

This keeps resp in it's own package.

Even if you don't merge this to master, making it a branch of github.com/mediocregopher/radix would allow people to reference that branch for at least some version of this repository.

It's understandably more ugly to have that /v3/ folder there, but it seems like the right approach while the Go community is in this transition period between dep and modules.