Also switch back to Maven Central repository.

This is a fairly functional variation of Gitblit with one notable
exception:

    The security filters are not working properly.

This is a design flaw in Guice that I have reported upstream [1].  The
general idea is that Guice-Servlet filters are not properly wrapping the
ServletRequest. This has historically been a problem for Guice-Servlet
servlets but Google has fixed most of those issues.  Unfortunately, all
the same flaws reported against the servlet delegation also exist in
Guice-Servlet filter delegation. :(

[1]: https://code.google.com/p/google-guice/issues/detail?id=807

The Jetty 9.2 quick start mechanism provides almost an order of
magnitude improvement in start time [1].

[1] https://webtide.com/jetty-9-quick-start/

Plugins are stored in `${baseFolder}/plugins` and are loaded
during startup by the PluginManager.

A plugin defines it's metadata in META-INF/MANIFEST.MF:

    Plugin-Class: com.gitblit.plugins.cookbook.CookbookPlugin
    Plugin-Dependencies: foo, bar
    Plugin-Id: gitblit-plugin
    Plugin-Provider: John Doe
    Plugin-Version: 1.0

Plugins can define extension points that can be implemented
by other plugins and they can depend on other plugins:

    Plugin-Dependencies: foo, bar

During the load phase, a directed acyclic graph is built and the loading
order of the dependency chain is reversed using a topological sort;
parent followed by children. The parent plugin classloader is the
combined classloader of all parent plugins.

Change-Id: I738821fa2bff02a5dbe339a944cc7e3c4dd8e299

Change-Id: Ic7e204fb7ee2eb172f4c8424fc3d4b7e6c631ddd

Gitblit would greatly benefit from an integrated SSH server.  This would
complete the transport trifecta.

Change-Id: I6fb95abe65655fa74d47ea71522d8d9a1541450c