Upgraded Extract

This addresses multiple correlated issues. I'll ping back in followup comments. A major refactoring was done on this branch.

`data_preparation.py` is the current entrance point.
Take a look at the intermediate and final output: http://hupili.net/projects/hk_census/data/
I suppose this is what we need, except for some details. Two in my mind:

   * identifier (for database). 
   * [raw name to canonical name](https://docs.google.com/spreadsheet/ccc?key=0Asi0lKkzNhjsdENOYnVKaXE0RFR2VC1BOVcwM1lMZkE&usp=drive_web) created by @clacanzo . Need more people to review the mappings.

Suggest to followup the two on separate issues.