Organize the hook callbacks into logical groups.
Use time constants instead of multiplying integers.
Register hook callbacks at construct instead of camptix_init.

camptix_init is too late to affect things like camptix_default_options.
Allow addons to add custom e-mail templates.
Add custom e-mail templates for confirmed and unconfirmed attendees.
Allow addons to override e-mail templates.
Tweak the language of the custom e-mail templates.
Override the default e-mail templates with our custom ones.
Allow addons to modify the attendees shortcode get_posts() arguments.
Hide unconfirmed attendees from the attendees shortcode.

Fixes #585-meta.