Fix bug in `total_energy()` where all cache would be returned instead
of selecting relevant rows from cache. #216
Implement `ElecMeter.clear_cache()` and
`MeterGroup.clear_cache`(). Also:

* used `clear_cache()` in tests to make sure the test HDF5 files are
  left without a cache at the end of the tests.
* implemented a `HDFDataStore.remove()` function.
`HDFDataStore` now has `append` and `put` methods which operate as
their names suggest! #232
UK-DALE converter now follows same pattern as REDD converter. And uses
`put` instead of `append`.
Documentation now says that `MeterGroup.total_energy()` does not
return total aggregate energy.  Closes #148
added put and remove functions to CSVDataStore #230
CSVDataStore.load() now returns generator instead of text_file_reader
Added documentation for `HDFDataStore.__getitem()__`.  #230
added CSVDataStore.__getitem__()
Merge branch 'master' of https://github.com/nilmtk/nilmtk
Added documentation for . #230
Trivial white space and documentation tweaks.
Made a start on refactoring `ElecMeter` stats functions. Reducing
redundancy. Paving the way to refactor the code for using cached
stats. #216
Refactored `dropout_rate`
Refactoring code for computing and caching `ElecMeter` stats. #216
merging with Nipun's README