Stand up zippy support in solitude (bug 932557)

https://bugzilla.mozilla.org/show_bug.cgi?id=932557

Adds:

* a client for the Zippy API
* a mock of the client to launch tests without a Zippy instance
* give access to `zippy/{reference}/{resource}/` URLs to GET/POST and returns json

and a bit of https://bugzilla.mozilla.org/show_bug.cgi?id=932575

* give the ability to list/create a seller object against Zippy