Open up API to allow using token-map

Dear Datastax java-driver-team, in order to improve Hadoop support for Cassandra, i propose to merge this pull request. This will simplify/allow
* to remove the need for cassandra-all as a dependency for jobs, such that jobs only depend on java-driver and will be much more light-weight
* to page and cope with VNodes (which seems to be a problem either with the straight-forward way i used CQLRecordReader or the current implementation within cassandra-all)
* to remove the need for the Thrift-based call splits_ex, which was needed to calculate splits (with thrift being kind of deprecated...)

imho it is beneficial to open up java-driver to allow access to TokenMap, as included in the pull request. It is easy to prepare an InputFormat based on this, see https://github.com/AndreasPetter/cassandra-hadoop-vnodes .

Thanks for opening up this driver and for your help in advance, Andi