Add standard name resolution to the purge method (#64)
Add standard name resolution to the purge method (#64)

All methods in database manager treat `name` == `None` as
`name` == `self.get_default_connection()`  Purge almost works this way, except
for the `name` it cleans up from the `_connections` cache.

Fixes this behavior so that `None` cleans up cache entries properly
sdispater/orator,Hanaasagi/sorator