Add the ability to query the minimum or maximum values of a given column in query sets

[ellmetha]

Description

There is presently no easy way to query the minimum/maximum values of a given column - at the database level - by leveraging query sets. In order to palliate this, let's add new #minimum and #maximum methods to the base query set implementation. These new methods should take the name of a field for which the minimum/maximum value should be queried.

For example:

Person.all.minimum(:id) # => 1
Person.all.maximum(:id) # => 145012345

These methods should also be available from model classes directly:

Person.minimum(:id) # => 1
Person.maximum(:id) # => 145012345