cassandra 2.0 support
Better exception message.
Merge remote-tracking branch 'coursera/cassandra-20' into c2_support

Conflicts:
	aegisthus-hadoop/src/main/java/com/netflix/aegisthus/io/sstable/OffsetScanner.java
	aegisthus-hadoop/src/main/java/com/netflix/aegisthus/io/sstable/SSTableScanner.java
	build.gradle
Non-columnar tests passing against 1.2.18 input.
Add support for specifying SSTable version.

 - You can now specify the sstable version through the command line or through a job configuration property.
 - With this change all tests now pass again (currently this just means
   that we can still correctly process 1.2.5+ files with the Cassandra 2
   jars)