Indentation bug fix related to multi-line comments and multi-line strings.  Previously if a multi-line comment or string contained the word 'end', it would throw off the indentation level for all following code.

This normalizes the classes used by most of the modes
(reStructuredText and diff didn't fit in), and makes them all
reference a central style sheet (under theme/). It also adds a theme
option that can be set to change the theme, and defines a few
different themes (some based on old mode colors).