Deprecate wordpoints_display_admin_screen()

Fixes #132
Add headings to the top users table shortcode

Merges the avatar column into the username column. Adds CSS for making
the avatars still look nice.

See #131
Introduce wordpoints_points_show_top_users()

Fixes #131, #133
Introduce WORDPOINTS_URL constant

Fixes #135
Register admin scripts and styles

Fixes #124