Added an itemselector for hstore fields
Nice!!  Thank you.  Also thank you for the test case.

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.*'))
```
