Prepend should not require CAS

Hopefully this is just a doc bug and not in the actual code, but all of the atomic updates including prepend (http://www.couchbase.com/docs/couchbase-sdk-java-1.0/couchbase-sdk-java-update-prepend.html) should not require CAS. That significantly defeats their performance effectiveness.

If the function does require CAS, we should document that a CAS id of 0 will override the requirement to get the key again as in this example:
client.prepend(casv.getCas(),"samplekey", "prependedstring");