Lots of changes:
* cached stats now respects 'ac_type'
* stats can be passed 'ac_type'
* energy_per_period now has a `use_uptime` param
* energy_per_meter can be asked to provide average energy per period
  per meter
* can now produce nice rank of energy per meter per period
Merge branch 'master' of github.com:nilmtk/nilmtk

# Please enter a commit message to explain why this merge is necessary,
# especially if it merges an updated upstream into a topic branch.
#
# Lines starting with '#' will be ignored, and an empty message aborts
# the commit.