XSLT-based "Schema Browser" in admin view

The files in this upcoming patch create a simple "schema browser" for the admin tool. It serves schema.xml along with a stylesheet that in compliant browsers creates a page with a tree widget to show the fieldtypes and fields, as well as their uses and cross references. This is similar to the schemaxsl.zip originally attached to SOLR-58, but a few features have been added, and the look and feel has been changed to fit in better with the rest of the admin tool.

Note that it does not work against the live IndexSchema – it merely transforms schema.xml. There is probably not a significant difference now, but it is worth raising the issue in case there are future administration capabilities in mind (i.e. on http://wiki.apache.org/solr/MakeSolrMoreSelfService ) that might require a schema browser to be talking to the live values.