Add Relation Query

Now it support relation query:

	place = Place.Query.get(objectId="sscyMpfZJ2")
	categorys = Category_Place.Query.relation_filter(category__relation=place)._relation_fetch()
	print categorys

equal to :

	import json,httplib,urllib
	connection = httplib.HTTPSConnection('api.parse.com', 443)
	params = urllib.urlencode({"where":json.dumps({
       		"$relatedTo": {
        	"object": {
           	"__type": "Pointer",
           	"className": "Place",
           	"objectId": "sscyMpfZJ2"
         	},
         	"key": "category"
       	}
     	})})
	connection.connect()
	connection.request('GET', '/1/classes/Category_Place?%s' % params, '', {
       		"X-Parse-Application-Id": "MQRrReTdb...",
       		"X-Parse-REST-API-Key": "44mp6LNg..."
     	})
	resp = connection.getresponse().read()
	result = json.loads(resp)
	print result