Release of v2.3.1

This release has both bugfixes and several new features. Of the bugs fixed the most important one is the IE6 back-button bug, which required IE6-users to hit their back-button twice to go back a page.

There's quite a number of features added:
- Configurable DOCTYPE and Content-Type
- Default target for all items
- Possibility of not linking the text of submenu items without a URL when emulating Windows Explorer
- Storage of tracked item in a cookie
- Linked .js-file into the menu frame, with a configurable function to be called after loading
- Default values for all configurable options

Menu creation code has been simplified, there's no need to use the items[] array when building the menu.

Documentation is also updated to reflect the changes.