Feature/inflation adjustment

Add inflation support (via cpi data) to openspending via the economics python package. Inflation can be made visible via an inflate url parameter on an entry page and in the aggregation. Inflate takes in a date (but uses only the first four letters to get a year because of lack of more granular data). Inflation will not work if data is not found in the inflation data and the user will get a warning.

Data can be updated at http://github.com/datasets/cpi