Rename extension to ckanext.budgets instead of ckanext.budgetdatapackage

ckanext.budgetdatapackage is just too long and boring to type. It's
also nicer to call it budgets since that opens up the possibility
of including more budget related plugins that not only add Budget
Data Packages to CKAN.

Also, Budget Data Package is just a distrbution format so this move
is more like renaming something like "csv" to "tabular data"
because tabular data can be more than just csv and csv is just one
tabular data format.
Add README and LICENSE to the repository

These files should have been here from the start.