* implemented Electricity.get_start_and_end_dates
* Electricity.plot_missing_samples uses get_start_and_end_dates to set
  xlim
* wrote simple examples/load_hdf5.py
* prepared Electricity.plot_missing_samples for two modes: 'rectanges'
  and 'bitmap'
Merge branch 'master' of github.com:nilmtk/nilmtk

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