Adding indexes based on @jetherton's blog post to speed up sql queries

Adding indexes based on this blog post: http://ethertontech.com/2012/09/28/optimizing-ushahidi-for-quick-reads-on-large-datasets/
Most of these are obvious and apply generally. I've skipped
the full text index on form_response since that won't apply to all users.
Merge pull request #1018 from rjmackay/additional-indexes

Adding indexes based on @jetherton's blog post to speed up sql queries