REST-144. Separate out the /v1 endpoint into a plugin.

The standard plugin contains the v1 endpoint for KijiREST. Right now, it's
bundled into the core of KijiREST but could be split out in case users want to
have custom plugins within KijiREST but disable access to the /v1 endpoint which
may grant end-users access to all tables and instances within a cluster.

Reviewer: lsheng
Review: https://review.kiji.org/r/1969/