Add getitem operator to OPERATORS for postgres ARRAY

Hi.
I have the exception when without this path using sqlalchemy and postgres.
When I try to sort and limit query by an item of ARRAY field result is the exception:

in visit_binary
    OPERATORS[operator], **kw)
KeyError: <built-in function getitem>

for example:

session.query(Product).filter(SomeModel.array_field[2] > 500).order_by(SomeModel.array_field[3]).limit(60)
but all ok without limit.
This commit are solved this problem. 
Good luck.
Alexey Terentev