Adds a values_of class method to ActiveRecord::Base classes.

Under the hood is simply does a construct_finder_sql, forcing :select to be 
whatever you pass in the first argument, using that to call 
connection.select_values and (optionally) type_cast()ing the resulting values.

Aside from being prettier than passing a honking long SQL string to 
connection.select_values, the use of construct_finder_sql also ensures that
any with_scope() in effect will be honored.

USAGE:

values_of(colspec, options = {}, &block)

* colspec can be a:

Symbol - in which case you are selecting that column and (unless you pass a 
block) you want the in-built rails type-casting to turn the raw text sql 
result into the relevant type.

String - an sql expression you want to use for the :select portion of your 
query.  Note that if you pass a String then values_of won't try to automatically
type_cast your returned values.  See the info about &block below.

* options are anything normally accepted by ActiveRecord::Base#find.

* &block is an optional block that will be collect()ed on your returned string 
values.

Note that &block overrides any implicit type_cast()ing that would be done
if you pass a Symbol as a colspec.

EXAMPLES:

Project.values_of(:created_on, :limit => 10) 
# SELECT projects.`created_on` FROM projects LIMIT 10
# [Date, Date, Date, ... ]

Project.values_of('created_on', :limit => 10)
# SELECT created_on FROM projects LIMIT 10
# ["2006-01-24", "2006-02-21", ....]

Project.values_of("distinct DATE_FORMAT(created_on,'%Y-%m-1')", :limit => 10)
# SELECT distinct DATE_FORMAT(created_on,'%Y-%m-1') FROM projects LIMIT 10
# ["2006-01-1", "2006-02-1", "2006-03-1", ....]

Project.values_of("distinct DATE_FORMAT(created_on,'%Y-%m-1')", :limit => 10) do |raw_val|
  raw_val.to_date
end
# SELECT distinct DATE_FORMAT(created_on,'%Y-%m-1') FROM projects LIMIT 10
# [Date, Date, Date, ... ]

User.find(1).projects.values_of(:id)
# SELECT projects.`id` FROM projects WHERE (projects.user_id = 1)
[1, 8, 16, ...]