Several bug fixes and updates (Drupal 6)

* Caching did not work at all (filter made new files each time the node was displayed)
* CSS class for math, so you can change it from the default 'math' value
* Change quotes to double quotes in the img tag (to pass HTML validation)
* Short math quotes (Example: [$x=x^2$])

Now filter works perfectly with codecogs (http://www.codecogs.com/components/equationeditor/equationeditor.php) and short tags are much better for me.