Lots of changes relating to plotting (#201) and loading data:

* removed 'start' and 'end' parameters from plot. Instead use
  DataSet.set_window
* power_series is no longer deprecated.  It's actually quite useful!
* replaced 'has_key' with 'in' in elecmeter.py #286
* reduced redundancy in plotting functions
* ElecMeter.load() now accepts `sample_period` and `resample`
  arguments to make life really easy for resampling!