Fixed Typo

Fixed a typo ("deriver" => "derived") in form.php.
Regex Fix

Regex used to split receipt notes on line breaks  did not have the "u"
flag and was therefore not in Unicode mode.

The flag has been added.
Wording Change -- Login Page

Changed the wording on the checkbox which keeps users logged in to
"Remember Me?" which seems more natural.
Schema Change -- Membership Types

Added two new fields to the `membership_types` database table:

1. `show` controls whether or not a particular membership type will be
an option on the membership application form.

2. `order` controls the order in which membership types will be
displayed on the membership application form.
Membership Application Beautification

Applied Jason's requested cosmetic changes including reorganization of
the form (membership types above organization info instead of vice
versa) and more headers to further break the form out into sections.

Each membership type is now displayed alongside its description and
price.

Membership types are sorted based on the new `order` field in the
`membership_types` table, and membership types with `show`='0' are not
shown (and will not pass back-end validation).