uri-building enhancements

[nihiliad]

Creating a new plugin for WorldCat Discovery exposed a couple of incorrect assumptions in the design of Janus:

• An HTTP request for the baseUri should always return 200.
• The emptySearchUri will either be identical to the baseUri, or will extend the baseUri in the same way as search result URIs.

These enhancements will correct the above assumptions make Janus uri-building more flexible:

• Add a new, overrideable searchBaseUri() function to plugin.js, which returns the baseUri by default.
• Remove the integration test for baseUri, which expects a 200 response, from plugin-tester.js.