Csvdatastore update

This pull request is a combination of old #652 and #656 - due to the nature of the changes, neither of them will work separately (they will cause the build to break). As such I've made this pull request with each set of changes as one of the commits. The first commit is the backport of geoserver's updates to the gt-csv module. The second is an update to the ContentDataStore tutorial to reflect these new changes. A final update may be needed for licensing changes.