beedb.go: use limit/offset syntax when constructing sql

Traditionally in database management systems, one uses "LIMIT #,#" to slice rows on a select statement. Many database management systems also support a synonomous API using "LIMIT # OFFSET #" to achieve the same thing. Some database management systems have decided to disable the former method of slicing rows. For example, PostgreSQL disabled support for "LIMIT #,#" in version 7.3 ([release notes](http://www.postgresql.org/docs/devel/static/release-7-3.html)). Versions below 8.3 for PostgreSQL are so old that they are now unsupported. This means that at the moment, `beedb` does not support slicing rows for any recent version of PostgreSQL.

This change updates `beedb.go` to use the "LIMIT # OFFSET #" query API. The change restores slicing functionality for more recent versions of PostgreSQL while hopefully not breaking any functionality for any other database management systems. As far as I know, they all support this syntax.