cache the number of patrons in the participants table
remove `Participant.get_number_of_backers()`
update tests
initialize the `npatrons` column
rewrite `iter_payday_events()` and increase its test coverage
Merge pull request #2573 from gittip/more-history-tests

Simplify iter_payday_events and increase its test coverage
Merge pull request #2554 from gittip/cache-npatrons

Cache the number of patrons in the participants table
merge branch.sql into schema.sql
Bump version to 14.0.8
Bump version to 14.0.8-dev
use temporary tables instead of making lots of expensive queries
replace `get_tips_and_total()` and `get_takes()`
remove unused arguments
factor record_charge and record_credit
remove monkey-patching
rename argument
move exceptions from withdraw_balance_to_bank_account to ach_credit
use the same exceptions in charge()
create new module gittip.billing.exchanges
update and move tests