time_zone_select helper enhancement

As we all know it, Time Zones are a pain in the neck. Rails make
things easier by providing some nice helpers. However, I recently
faced a challenge when trying to pre select a time zone for a user.
Using the time_zone_select helper, there's simply no way you can pre
select a timezone for a user unless you already saved the time zone in
his/her record. Creating a new user or editing a user with an optional
timezone, becomes quite a challenge.

That's why I created this patch:

http://dev.rubyonrails.org/ticket/10590

The patch adds support for a :default option allowing to select a
default time zone if the user's time zone isn't set already.

I also added updated the help with usage examples.

Nice patch; applied. Thanks Matt.

jeremy