Experimental Cassandra 2.0 support

This commit does not preserve backwards compatibility with Cassandra
1.2.  It's a quick feasibility test for 2.0 client-side support.

* Invasive source changes are required in the embedded adapter, in
  particular with explicit timestamp handling, but the embedded tests
  pass

* Thrift adapter required only one small change: a public static
  String field storing the name of the system keyspace in a Cassandra
  internals class has moved

* Astyanax adapter is broken with no apparent upgrade path.  Astyanax
  does not yet support Cassandra 2.0 on the client side, except
  perhaps in the unreleased beta Java driver branch.  Linkage errors
  at runtime when running against 2.0

* The Guava version is delicate.  Going to 16.0 or newer breaks
  Cassandra 2.0.x, which was compiled against Guava 13.x.  16.0
  changed the return type of
  com.google.common.util.concurrent.RateLimiter.acquire(int) from void
  to double, and that causes linkage errors with Cassandra at runtime.
Merge branch 'titan05' into experimental-cassandra-2.0

* Resolved conflicts around the migration from time variables of type
  long to time variables of type Timepoint or Duration

* Deleted the Astyanax adapter, since the current release of Astyanax
  is incompatible with Cassandra 2.0's Thrift version and the
  2.0-compatible javadriver-based rewrite of Astyanax is in beta

Conflicts:
	titan-cassandra/src/main/java/com/thinkaurelius/titan/diskstorage/cassandra/embedded/CassandraEmbeddedStoreManager.java