simplify and fix the SQL queries in /about/stats

closes #1770
use data cached in the paydays tables instead of recomputing it
fix query to ignore one-off tips that happen during payday
Export path to avoid need to activate virtualenvin
Merge pull request #2635 from gittip/plain-payday

Don't require activating virtualenv for payday
Merge pull request #2633 from gittip/fix-stats

Fix stats
Bump version to 15.0.3
Bump version to 15.0.3-dev
Merge branch 'master' into i18n
Merge pull request #2599 from gittip/i18n

Internationalize user interface messages
Regularize the locale code
Merge pull request #2638 from gittip/improve-i18n

Regularize the locale code
Bump version to 15.0.4
Bump version to 15.0.4-dev
Merge branch 'master' into gratipay