Refactors BaGet:

1. Moves business logic out of controllers and into BaGet.Core. For example, the PackageContentController passes requests to IBaGetPackageContentService and maps results to HTTP responses. In effect, controllers are just translate HTTP translators.
2. BaGet's services for the NuGet protocol implement the same interfaces as BaGet's client libraries for the NuGet protocol. For example, AzureSearchService and SearchClient both implement ISearchService
3. Requests and responses are codified by models in BaGet.Protocol. For example, both SearchController.SearchAsync and SearchClient.SearchAsync return a SearchResponse