Reduce redundancy between metergroup.combine_chunks_from_generators
and MeterGroup.dataframe_of_meters. Closes #291. Also:

* `load` and `dataframe_of_meters` both now take a `resample`
  parameter to optionally resample before reindexing.  Defaults to
  False.
* Replaced all `has_key` with `in` in `metergroup.py`. #286
* renamed `_check_kwargs` to the more descriptive name
  `_check_kwargs_for_full_results_and_sections`
* made some docstrings conform better to Numpy docstring standard.