Finish documenting the functions in renpy.exports.
Word-like operators have to end at word boundaries.

This prevents us from parsing strings like "8 insensitive" as "8 in
sensitive", and then failing when we go to eval it.

Fixes http://lemmasoft.renai.us/forums/viewtopic.php?f=32&t=25510
Rewrite missing images in terms of screens and styles.

This makes it possible to customize the missing image indicator,
which is useful if it conflicts with the background of the game.
Bump version.