few new features

* functions.Token (http://cassandra.apache.org/doc/cql3/CQL.html#tokenFun), usage: 

    query.filter(pk__token__gt=functions.Token(pk))
* QuerySet.values_list (`flat` argument is also supported)
* do not call .count() inside QuerySet.\__len\__ - len(query) is called for example when list(query) is called, resulting with additional "select count(*)" statement