fixes #185: Opened up Traverson for custom LinkDiscoverers

Opened up Traverson to pick up LinkDiscoverer beans via a LinkDiscovererPluginRegistry when configured as Spring bean, e.g. via EnablePluginRegistries or EnableHypermediaSupport. See TraversonDefaultJsonTest.ContextConfiguration for an example.

Introduced DefaultLinkDiscoverer which finds links in the default json format. Added DefaultLinkDiscoverer to the link discoverers that are available when using new Traverson().
Renamed Server to HalServer since that is what it talks, factored out DefaultJsonServer as parent of HalServer which talks application/json.
Traverson.prepareRequest merely wrapped the passed headers into a HttpEntity without adding Accept to it, unit test failed to detect that.