Read IndexSchema from InputStream instead of Config file

Soon to follow patch adds a constructor to IndexSchema to allow them to be created directly from InputStreams. The overall logic for the Core's use of the IndexSchema creation/use does not change however this allows java clients like those in SOLR-20 to be able to parse an IndexSchema. Once a schema is parsed, the client can inspect an index's capabilities which is useful for building generic search UI's. ie provide a drop down list of fields to search/sort by.