add configuration options to control Cassandra compression
Making Cassandra compression algorithm configurable via two new configuration properties (storage.sstable-compressor and storage.compression-chunk-length)
Merge branch 'issue_379'

Conflicts:
	titan-cassandra/src/main/java/com/thinkaurelius/titan/diskstorage/cassandra/astyanax/AstyanaxStoreManager.java
	titan-cassandra/src/main/java/com/thinkaurelius/titan/diskstorage/cassandra/thrift/CassandraThriftStoreManager.java
Merge branch 'master' of https://github.com/mamccr/titan into mamccr-master

Conflicts:
	titan-cassandra/src/main/java/com/thinkaurelius/titan/diskstorage/cassandra/AbstractCassandraStoreManager.java
	titan-cassandra/src/main/java/com/thinkaurelius/titan/diskstorage/cassandra/astyanax/AstyanaxStoreManager.java
	titan-cassandra/src/main/java/com/thinkaurelius/titan/diskstorage/cassandra/thrift/CassandraThriftStoreManager.java
Refactor Cassandra's sstable_compression config

* Renamed SSTABLE_COMPRESSION in AbstractCassandraStoreManager to
  COMPRESSION_KEY to match the existing pattern of public static final
  String config keys in that class

* Added psfS DEFAULT_COMPRESSION set to "SnappyCompressor" instead of
  passing the literal directly to the config parsing code

* Changed user-visible "sstable-compression" configuration key to
  "cassandra.db.compression.sstable_compression" to match the pattern
  of the other two Cassandra-compression-related config keys.  I think
  having "Cassandra" is definitely redundant, and "db" is probably
  redundant, so I may change this again.
Refactoring CF compression in embedded Cassandra

* Added support for disabling compression entirely in the embedded
  cassandra adapter per the config key introduced in #379

* Removed code that instantiates a compressor class from the embedded
  store manager, instead passing a string to Cassandra's
  CompressionParameters constructor.  The constructor handles
  reflective instantiation on our behalf.  This is an improvement
  because CompressionParameters contains logic to prepend
  "org.apache.cassandra.io.compress." to compressor strings that
  contain no dots, allowing shorthands like "SnappyCompressor".
  That's also our current default compressor string.

* Expanded debug log statements around compressor configuration in the
  embedded adapter
Renaming Cassandra compression config keys

Removed the "cassandra.db." prefix from these Cassandra configuration
keys:

* compression.enabled
* compression.chunk_length_kb
* compression.sstable_compression

Also: the middle option in the list was formerly chunk_size_kb.  I
changed it to chunk_length_kb because that's the string Cassandra uses
and I don't see a reason why the need to be subtly different.

I think the "cassandra" prefix is redundant because graph configs have
"storage.backend", which must be set to a one of the strings
identifying a Cassandra backend for any of these keys to be read by
Titan in the first place.  "db" is debatable.