Download and convert reference tables to create 4 additional datasets

@JeniT I've expanded the conversion to include a set of extra datasets. There are now 4 additional datasets, one for each of the worksheets in the PPI indices reference table (see `README.md`). These include current rates similar to those given [in this part of the bulletin](http://www.ons.gov.uk/ons/rel/ppi2/producer-price-index/december-2013/stb-producer-price-inflation--december-2013.html#tab-Output-prices--summary).

So, for example the phrase "Core factory gate prices rose 1.2%" can now be linked to an actual observation.

The observations use the same `cdid` codes as the main dataset so there's some correlation possible there too. There's also a `source` key which references the main dataset so we can identify derivatives.

I can expand this to include a little more historical data if we want to have some more examples?

A merge wld be appreciated!