Create zh-cn.po

starting a simplified chinese translation
Update zh-cn.po
Add money description strings
Merge branch 'master' into chinese
ignore escrow shuffles
reduce schema.sql
fix a few things
add missing index

It wasn't in schema.sql but we have it on the production DB.
fix `Payday.update_stats()`
fix `get_last_modified()`, the last modification is mtime, not ctime
Merge pull request #2649 from gittip/chinese

Chinese translation
add French translation
cleaner way of removing trailing zeros
Remove Esperanto from Chinese translation
Merge pull request #2678 from gittip/fix-zh-cn

Remove Esperanto from Chinese translation
Merge pull request #2668 from gittip/i18n-fr

French translation
Bump version to 15.0.13
Bump version to 15.0.13-dev
Merge pull request #2677 from gittip/fix-cache

Fix `get_last_modified()`
mv i18n/{zh-cn,zh}.po && make i18n_update