Correct id-collision check for new group creation for brand new
database table.

If the group database table is new, and thus there are no entries,
the check to make sure the group id doesn't already exist had
problems because it was checking for membership in a null array.
This is now fixed.
Streamline type selection for group creation on settings page.

Instead of three separate boxes, there is now only
one box plus a radio button selection. This has
made the code easier to read.
Comment out ability to add an individual-type group in settings page.

Since I'm not yet sure what type of functionality is wanted with
the individual-type group, I've removed the ability to add a new
individual group in the settings page, and instead just create
one individual group at activation.

This one individual group is given an empty name, so that
no further accidental adding of a group with an empty name
(as might happen if an admin hits enter on the settings page
without having typed a group name) can occur.
Add options table to database, and fix issue with tables in activation.

The activation script now creates an options table, for storing
some persistent options for the plugin. Also, it was discovered that
using SERIAL as a datatype was causing problems with the health data
table.
Implement ability to set payment and survey pages in settings page.
Implemented ability to look up proper 'welcome' link for a group.

On the settings page for the plugin, a list of the groups
defined in the database can be looked through, and the proper
group selected; once it is, the link that should be sent to
members of this group upon sign-up is shown to the admin user.
Refactor the sign-up-and-pay.php script so that only one page is
necessary.

Rather than having Paypal redirect the user to a separate confirmation
page, the script now redirects back to itself, and simply checks
the query_string for the token in order to determine if it's being
visited for the first time or as a confirmation.

Tested, and works, but may be brittle. Also, I really need to set
the pages via the database, rather than global variables as now.

This script will be improved significantly soon.
Stop support for consumer groups, and allow setting of "thanks" and
"error" pages.

Unfortunately, two things got tangled in this one commit:
1) Paypal strips query strings before redirecting customers
back to the confirmation page, so at the moment a consumer group
gid cannot be passed around using query string, meaning a generic
'individuals' group_id will have to be assigned to both consumer
groups and individuals. To reflect this, the admin page was altered
to keep an admin from creating a consumer group.

2) Options for setting an "error" page and a "thanks" page were
added to the admin page.
Use already-made gravity form rather than hand-rolled survey.

As a stop-gap, I'm going to simply use a gravity form I already made
rather than put together a new form.