[all][m]: improve script in variety of ways.

* correct script to use changed nationwide source url
* Pull out percentage changes
* Round numbers to correct decimal places
* Change to iso formatted dates rather than float date
[refactor][xs]: relocate files to standard locations.
[dp.json][xs]: add graph.
Merge branch 'master' of github.com:datasets/house-prices-uk

Conflicts:
	data.csv