PCodec DateTime impl error

[Derek Troy-West]

Using latest Alia, and Cassandra 2.0.10:

The current PCodec DateTime implementation returns a Long, where it should return a standard java Date. Currently persisting a joda DateTime leads to:

<InvalidTypeException  com.datastax.driver.core.exceptions.InvalidTypeException: 
Invalid type for value 3 of CQL type timestamp, expecting class java.util.Date but 
class java.lang.Long provided>

I'll raise a PR one second.