Partially implement API changes to webservice

Implement API changes to add-context-handler and add-ring-handler.
Finish Handler API Changes

Implement API changes for add-servlet-handler, add-war-handler,
and add-proxy-route.
DRY up options schemas

DRY up options schemas by putting the server id option in its own
schema, then associating other options into it as necessary for
other schemas.
Finish implementing API changes

Implement API changes to all remaining service functions
(override-webserver-settings!, get-registered-endpoints,
log-registered-endpoints, and join).
Address comments on API changes

Address the PR comments on these API changes. These include
minor clean-ups and fixing an issue with the join function.
Simplify acquisition of server context

Simplify the acquisition of a server context by allowing the
get-server-context function to take nil as a server-id, in which
case it will pull the server context for the :default server.
Merge pull request #40 from fpringvaldsen/Jetty9-API-Changes

Jetty9 API Changes