A few improvements related to composite primary keys

Hi, I am using this project for a technology demo combining Cassandra 2, Spring 4 and Tomcat 8.

A few things did not work for me so I thought I would share my changes:

findByPartitionKey implementation assumed a single primary column. I added a new getPartitionKey method to the CassandraConverter.

CassandraPropertyValueProvider does not support some Casandra types, like UUID, because the Datastax driver implementation of Row.getBytes requires the column dataType to be BLOB. Replaced a call to getBytes() with a call to getBytesUnsafe().

MappingCassandraConverter would skip over the embedded primary key properties. I fixed the problem but perhaps not in an elegant way.

Hope you will find this useful.