[Fixes #89] Reworking content retrieval component chain to inject a new layer - ContentManager - that manages both DownloadManager (was: FileManager) along with a new class of component called ContentGenerator. The ContentGenerator interface supercedes the old GroupPathHandler, and is generalized to handle group content-merging and content generation for remotes that are missing it. Using this new approach, maven-metadata.xml is generated for remote repos that don't have it. Also, consolidating all the access classes into a single REST resource with generalized routing patterns. Finally, fixing some bugs in content browsing related to presence/absence of index.html for a directory when requesting the directory listing.