implement conditional puts #18

possibly using strong consistency & quorum reads and writes.

https://github.com/basho-labs/rinamo/blob/master/src/rinamo_items.erl#L14-L20

• should fail put item if conditional expectation fails *** FAILED ***
  Expected exception com.amazonaws.services.dynamodbv2.model.ConditionalCheckFailedException to be thrown, but no exception was thrown. (HashMappingTest.scala:125)