Add serverInfoRequest and serverInfo types

In #36, there is a desire for a mechanism to get information about how the ACME server will process requests. This PR adds a mechanism, which could also be helpful in automated discovery of ACME servers in conjunction with DNS `SRV` records and perhaps help reduce reliance on pre-populating a list of available servers in ACME clients.

To accomplish the specific request listed in #36, a [JSON Web Key Set](http://self-issued.info/docs/draft-ietf-jose-json-web-key.html#JWKSet) is used. This allows a server to specify as many signing certificates as it may use, perhaps supporting different signing algorithms on the point of #36.

Feedback and suggestions requested!