Implementation for no binary content jcr/xml file system persistence.

https://www.pivotaltracker.com/s/projects/684825/stories/74702010

We should be able to persist jcr/xml with no binary contents to the file system now. The location can be configured with parameter jcr.persist.dir as defined in in fcrepo-jms-indexer-webapp/src/main/resources/spring/indexer-core.xml, and the filename for the jcr/xml will be {id}-jcr.xml.
Create a default value that is more suitable for production use (${java.io.tmpdir}?)