SolrInfoMBeanHandler - replacement for stats.jsp and registry.jsp

stats.jsp is cool and all, but suffers from escaping issues, and also is not accessible from SolrJ or other standard Solr APIs.

Here's a request handler that emits everything stats.jsp does.

For now, it needs to be registered in solrconfig.xml like this:

    <requestHandler name="/admin/stats" class="solr.SystemStatsRequestHandler" />

But will register this in AdminHandlers automatically before committing.