[exim] Wiki Restructured

I've restructured the Wiki - especially the front page. Nothing has been
deleted but several things are moved or merged and some dead end stuff
that was never finished was removed. I think this setup will be a lot
easier to use not only for finding stuff but for people to figure out
where to add new stuff. The spam filtering section is ripe for people to
add spam filtering tricks to.

http://www.exim.org/eximwiki/FrontPage

Let me know what you think of it.