added webservice methods to retrieve performance data

Not ready for merge yet. Just creating for review.
We should avoid using json here as a parameter value in a url. Try using django's ``getlist()`` function instead, ``request.QUERY_PARAMS.getlist("signatures")``. If the django REST framework doesn't expose ``getlist`` on ``QUERY_PARAMS`` then we should just use ``request.GET.getlist("signatures")`` directly. This will simplify some of the following code and allow you to completely avoid the ``ast.literal_eval()`` call.


https://docs.djangoproject.com/en/dev/ref/request-response/#django.http.QueryDict.getlist
http://stackoverflow.com/questions/3910165/problem-with-django-request-get-and-multiple-variables-for-the-same-parameter-na
http://www.django-rest-framework.org/api-guide/requests