Changed the type returned by `count` from `Int` to `Int64`

The PostgreSQL type of `count` is `bigint`
(http://www.postgresql.org/docs/9.1/static/functions-aggregate.html),
which `postgresql-simple` can convert to `Integer` or `Int64`, but not
`Int`.
Merge pull request #73 from c-lewis/fix_count

Changed the type returned by `count` from `Int` to `Int64`