Add chunk support for QueryBuilder

[Sébastien Eustace]

The QueryBuilder class should support chunks to ease on memory consumption for large results sets:

with db.table('users').chunk(100) as users:
    for user in users:
        # ...

[Sébastien Eustace]

Actually, it will not be a context manager but rather a generator:

for users in db.table('users').chunk(100):
    for user in users:
        # ...