Wrote custom 403/404 pages.

Both feature enhanced help.
Throw 403/404 errors instead of die()ing

Use the custom error pages to handle invalid requests. Fixes #11.
Provide different titles depending on page.
Added recommendation to README to compile .htaccess files
Preview tags in tag panel without asking server

We use pure JavaScript now.
Rewrote tag list logic in clean JavaScript.

The server doesn't have to render the tag list after every modification, we only
ask him about the tags to be added.
Pass tag object to writeTag(), enabling JSON input
Draw static tag lists using JavaScript.

We don't care any more on the server side about how tags look. They are
entirely drawn on the client side.

We also improved some of the query code in tasklist.php.