[archive][s]: add cache of old-style national xls.

* This was last download of national xls (CSUS, quarterly) including data up to 2013-Q3
* This type of file is no longer available for download afaict as national data now bundled with the city data (see upcoming commits for more)
[all][l]: major update both to scripting and to data.

scripts: handle constantly changing source data urls and different source data format.

* new find_links method to find the download links on S&P website (they change
  every release afaict)
* cache all the raw source data into archive
* refactor extraction as source data has changed. source data files have
  changed and national data now provided monthly along with cities in one
  single xls (archive/home-price-index-levels.xls). As such just have one output file

data: consolidate data in one file and get latest data

* remove separate national data files (see previous item in scripts)
* cities-month now has national data in last column plus last year of data
  (plus some changes to old data - weird!) - git diff --word-diff --color-words
  is your friend!

datapackage.json: as per data

README.md: remove obsolete data links and add some excerpted info on methodology etc

archive: add the raw xls for home-price-index-levels
[README][s]: add license section.