Provide a service that returns archive of Arc contracts

[dkent600]

We need to provide a service by which arc-js applications can obtain information about current, prior and future daostack contracts.

Information about future contracts requires a separate external server/source that we may defer doing for now.

For more information, see the prior related issues from the arc repo: https://github.com/daostack/daostack/issues/138 https://github.com/daostack/daostack/issues/139 https://github.com/daostack/daostack/issues/90

[dkent600]

Seems related to #3