Modifying query module to allow to compare by multiple keys in gt,lt,gte...

...,lte queries

Hi,
 A couple of weeks ago there was an email thread in which I was encouraged to submit this patch for review. 

I was needing to filter by multiple keys in the HStore field, something like

Something.objects.filter(data__gte={'a': '1','b':'5'})

This yields an Invalid Value exception. Looking at the code only one key/value pair could be passed inside the dictionary for gt/gt/lte/lt. 

I modified this in query.py and wrote the corresponding test. Please let me know your thoughts.

Thanks
Marcela
add a space after the comma;
is it possible to get rid of the extra parenteses?
```python
return (" AND ".join(conditions), param.values())
```
should be enough.