Initial work on issue/34

The issue is to allow specifying not only the number of documents to be
uploaded, but also the byte size of the documents. This is more
meaningful in terms of performance measurements.

The big architectural change is on how data is being fed into the
benchmarker - now it is an iterator of iterators - an iterator which
yields batches, which in turn yield individual documents.
Continued work on issue #34

Things look good, tests passing, better comments. Now just need to sleep
on this.
Better logging around sizes / counts of inserts.

Accurate counts / sizes of incoming data now logged. Turns out that the
size of the index is not the same as the size of the raw data that is
being inserted - the precise relationship is yet to be established, for
now the logging at least gives an idea of what is going on. I'd go out
on a limb and say that this closes #34.