Prepend a string to an existing key.
var success = client.prepend( "my-key", "again " );
// Prepend with previous operation
GetResult result = client.get_result("my-key");
success = client.append( "my-key", "again ", result.cas );
|cas||Optionally, the CAS as retrieved from a get_result() operation|
|true if the command was successful|