Go: Credentialset loading

[technosophos]

We need a pkg/credentialset that can read credentialset definitions.

The credentialset spec is here: https://github.com/deis/duffle/blob/master/docs/201-credentialset.md

In this first step, we really just need the unmashalling and the support for $HOME/.duffle/credentials.

I'll create a separate issue for resolving credentialsets into a list of credentials.