Additional Titan MR index support

Expanding implementation of the Mapper class that will support index
repair over Hadoop MR:

* TitanIndexRepairMapper now opens and closes a graph instance in
  setup and cleanup, respectively

* Renamed the "output.location" and "output.overwrite" keys to
  "tmp.seq.location" and .overwrite to better reflect their role in
  temporarily storing intermediate sequencefile data passed between
  jobs that Faunus/titan-hadoop chains together

* Added a SchemaStatus sanity check in TitanIndexRepairMapper