Improved and fixed DataFrame client

While using it a little more, I stumbled upon some issues with the DataFrameClient that I solved here:

* fixed a bug with handling time zones
* fixed a bug in ``get_list_series`` and ``get_list_continuous_queries`` (I've separated the ``query`` method in `InfluxDBClient` into a private method that returns always dict, and a public method that returns dict for `InfluxDBClient` and Dataframe for ``DataFrameClient``. That new private method should be used for not yet implemented methods like `get_list_database_admins` as well.)
* added support for writing DataFrames in batches
* added support for reading empty result sets
* added sorting of DataFrames