influxdb.misc.DataFrameClient -> influxdb.DataFrameClient

@timtroendle 
I tought you might like this better than the current setup.
I think this is a clever way of making pandas optional while improving the import structure.  It's also pypy-compaible since pandas can't be installed there. 

We haven't yet released a version of the lib with the DataframeClient so it does not break the API. I plan on making a release soon.