Implemented proportion_of_energy_submetered(). #31. Also:

* stub for `average_energy()`
* stub for `average_energy_per_appliance()`
* use consts for DEFAULT_MAX_DROPOUT_RATE and
  DEFAULT_ON_POWER_THRESHOLD
* changed names to get_sample_period and get_dropout_rate; and both
  of these functions can now take either a DataFrame or Series or
  DatetimeIndex
* added `periods_with_sufficient_samples()`
* added `_get_index(data)`