Contributions where Contributors can Select a Contribution Type

This is a patch of some functionality I added for our client. It is my first contribution to the community, but it works well on our site. It probably needs some work before being used in lots of sites, but it's a nice feature that some clients probably would like.

To get this to work, you have to change the database schema such that the "contribution_type_id" of the "contribution_page" table allows a NULL value.

Thanks!

Dan Ficker