add __ne__() function so that timeframe != timeframe works.
Implemented a test which I think tests for issue #133
I think this solves issue #133.  Appears to make it slow though.

The problem was caused by the fact that
we cannot simply test if len(data) == chunksize
because store.select(chunksize=chunksize) doesn't
appear to respect the chunksize argument!

TODO: report bug to Pandas / PyTables

Alternative approach: before this loop,
make a copy of the generator using tee()
and loop through it to find out how many
items there are in it, then count through those
in this loop.
Merge branch 'master' of github.com:nilmtk/nilmtk

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