Added index to hzrdi.hazard_site

Recently, a performance issue has been encountered (see http://monitoring.openquake.org/performance/jobs-2013-08-01.html ). This pull request addresses such problem by adding a database index (which does not add a significant overhead, as such table is populated once during pre-execute).

The query time on wilson.openquake.org dropped from 20 seconds to 406 milliseconds.