Added rhel support, plus some additional improvements

The main feature is Rhel support.

Other changes:

- new attribute template_cookbook - allows you to easily override templates location
- added java recipe to datasax.rb so you could have an option to update java version in use. That could be especially useful for cassandra, since you may need Oracle JDK
- create required dirs based on attributes ( if you changed data_root or commit_log dir for example )
- move templates before service start, especially useful for first runs on new boxes
- updated JVM_OPTS="$JVM_OPTS -Xss256k" to 256k to fix problem with Java 7 (http://comments.gmane.org/gmane.comp.db.cassandra.devel/7940). Cassandra just wont start without this change.

Hope you find it useful and merge it in, so we don't have to maintain.