added webservice methods to retrieve performance data

Not ready for merge yet. Just creating for review.
The ``SUM`` aggregate function in SQL returns ``NULL`` if the return set has no rows. The ``SUM(`property`=%s AND `value`=%s) > 0`` clause should probably test for that explicitly.

If you really need 0 I think you can use the ``COALESCE`` function to do that like so:
``COALESCE( SUM(`property`=%s AND `value`=%s), 0 ) > 0``

 You can also explicitly test for ``NULL`` in MySQL with ``IS NULL`` or ``IS NOT NULL`` operators. ``SUM(`property`=%s AND `value`=%s) IS NOT NULL``

Nice use of the ``HAVING`` clause in the SQL statement!

http://dev.mysql.com/doc/refman/5.0/en/group-by-functions.html#function_sum
http://dev.mysql.com/doc/refman/5.0/en/comparison-operators.html#operator_is-not-null