Fixes #79: Handle nulls in StringConverter
Merge pull request #80 from RussellSpitzer/master

Issue #79: Handle null values in String Converter
Regression test for issue #79 (null value handling in CassandraRow#toString)
Added changelog. Renamed LICENSE to LICENSE.txt.
Added object type to the error message produced by TypeConverter.
Merge branch 'master' of https://github.com/datastax/spark-cassandra-connector into issue-32
Added ChainedTypeConverter serializability test. Documented custom converters.