Added an itemselector for hstore fields

I added an itemgetter to the hstore field, which returns an expression the selects the corresponding key from the hstore. Now you can do stuff like:

    Test.select(Test.data['key'])

and

    OP_MATCH = 'match'

    def match(left, right):
        return Expression(left, OP_MATCH, Param(right))

    PostgresqlExtDatabase.register_ops({
        OP_MATCH: '~*',
    })
    
    Test.select().where(match(Test.data['key'], 'some regular expression.*'))