[IMP] fields.boolean: simplify symbol_set, cast to bool

In combination with f28be81, this should help speed up
initialization of new boolean columns. psycopg2 handles
bool parameters values just fine inside cr.mogrify()
[FIX] account, point_of_sale: fixed the feature to put money in or take money out of a cash register. Fix #1416
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-win32