Lots of changes relating to proportion_of_energy_submetered #104

* re-wrote proportion_of_energy_submetered
* implemented preprocessing.electricity.building.mask_appliances_with_mains
* fixed problem in preprocessing.electricity.single.insert_zeros where
  it wouldn't insert a zero if the datetime index already exists
* implemented get_total_energy_per_dict
* a few docstring tweaks
Merge branch 'master' of github.com:nilmtk/nilmtk

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