SolrCoreAdmin nonfunctional

SolrCoreAdmin as far as I could tell is completely non functional as it stands currently.  None of the methods work as written and all return the same result as you would get for /solr/admin/cores with no parameters.

This patch makes all SolrCoreAdmin methods functional, adds the missing data_dir parameter to create, and implements tests that actually assert that the SolrCoreAdmin is doing anything at all.