Make wiki page tables look nice

[leafstorm]

This is accomplished by adding the .table class to the tables, and maybe some other classes as well. (The available classes are listed at http://bootstrapdocs.com/v2.0.4/docs/base-css.html#tables. We probably want striped and maybe also bordered.)

We can achieve this by creating a custom subclass of TableElement (like we're doing now with the code block) that overrides _build and adds a .table class to the returned genshi.builder.Element.