*: add a new OpenID Connect client

[ericchiang]

Largely upstreamed from github.com/ericchiang/oidc.

Tests to come.

Please review the package API before specific pieces of code.

git clone https://github.com/ericchiang/go-oidc.git $GOPATH/src/github.com/coreos/go-oidc
cd $GOPATH/src/github.com/coreos/go-oidc
git checkout new-client
godoc -http localhost:8080

Then visit http://localhost:8080/pkg/github.com/coreos/go-oidc/

cc @squat @rithujohn191

[ericchiang]

Fyi this is not urgent and a work in progress.

[ericchiang]

@squat @chancez @sym3tri @rithujohn191 this is what the new API looks like https://godoc.org/gopkg.in/ericchiang/go-oidc.v2

[ericchiang]

Just going to merge. We can address any further issues as they come up.