Merge pull request #218 from odoo-dev/master-ormcache_new_api-rco

[MERGE] ormcache: improve API to enable the decoration of new-api methods

The new signature of `ormcache()` is more intuitive: simply list the names of the method arguments that compose the cache key. The decorators `ormcache_context()` and `ormcache_multi()` have been adapted accordingly.