Added new Java files to Makefile.am.
Handling paths for HttpRpcPlugins.

Makes sure plugins specify valid paths and are canonicalized correctly
before being added to the commands map.
Use RpcPluginManager.isHttpRpcPluginPath().

This is instead of the old, hardcoded way.
Added TestRpcPluginsManager to build.
Updated comments on HttpRpcPlugin.
Made accessor methods package-private.  Added collectStats().
TSDMain is responsible for RpcPluginsManager lifecycle.