rewrite

[ungoldman]

similar rewrite to https://github.com/koopjs/koop-github/pull/28

Fixed

• controller: preview route dependencies work on non-root mountpaths
• model: local cache works (resolved issue from koop upstream)
• model: github access tokens now work as expected

  Changed

• provider: name is now gist instead of Gist
• provider: status.version moved to version
• controller: using koop-provider's ctrl.errorResponse method for API error responses
• model: simplify find method (use options object)

  Added

• model: debug information for github api and koop cache
• model: looks for KOOP_GIST_TOKEN environmental variable if config.ghtoken isn't specified
• controller/routes: added rate_limit route for checking github rate limit status
• controller: support for JSONP callbacks (res.jsonp)