Rename "role" as "list"

The "role" table, and all the nomenclature surrounding it was really
inaccurate. The name should always have included some sort of email
reference, as the original goal was to support multiple simple mailing
lists.

This change renames the table and all the text and functions that refer
to roles.

Version number bumped to v 0.31
Make functions in Ajax tree protected not private

With me being new to PHP, I didn't realise that "private" methods are
not visible to subclasses. They are not visible, hence the switch to
protected -- I *do* want to reuse the code!
Add active flag to member table

The active flag will be used to indicate whether a user should continue
to receive email. In some cases (including my primary use case) we want
to hang on to the contact information, so we don't want to simply delete
the user. Actual manipulation of the new value will occur with changes
to the Members admin UI.

Version bumped to 0.32.
Clean up AdminAjax

The naming here got very messy, so I cleaned it up. All AJAX methods now
have the suffix "_ajax". I also made sure that all methods use the same
variable name for the return value, i.e. $response_data. There were
previously also uses of $responseData, and there were also some
copy/paste bugs that crept in where the wrong variable was used in a
snippet of code.
I also noticed that get_season_names() (now get_season_names_ajax())
didn't declare the $wpdb global, and fixed that as well.
Implement initial member management

Actually implement the interfaces and UI for managing members. This
includes a change to generalize the classes and CSS for tables in the
admin pages. It also implements the code to allow members to be
automatically enrolled for specific mailing lists, as well as
controlling whether mail can be sent to all active members.

Includes a version bump to 0.33 for new columns in the list table.