throw Unsupported exception instead of returning null in meta data getter for unsupported meta data.
Merge branch 'embedded-ttl' of https://github.com/thinkaurelius/titan into embedded-ttl

Conflicts:
	titan-cassandra/src/main/java/com/thinkaurelius/titan/diskstorage/cassandra/astyanax/AstyanaxKeyColumnValueStore.java
	titan-cassandra/src/main/java/com/thinkaurelius/titan/diskstorage/cassandra/thrift/CassandraThriftKeyColumnValueStore.java