Support serving static/ and Documentation/ from plugins

The static/ and Documentation/ resource directories of a plugin can be
served over HTTP for any loaded and running plugin, even if it has no
other HTTP handlers. This permits a plugin to supply icons or other
graphics for the web UI, or documentation content to help users learn
how to use the plugin.

Change-Id: I267176cc76e161617d780438f88531fc50c1c2b8
4 files changed