add PageDown plugin

Here's a [PageDown](http://Code.Google.com/p/pagedown/wiki/PageDown) plugin I put together based on the [WMD](http://WMD-Editor.com/) plugin source. In `editpage.tmpl` I moved `<TMPL_VAR WMD_PREVIEW>` to the top of the template. Makes it look like the edit `textarea` pops up below the content when hitting the edit link. Should work the same for the WMD plugin as well.

I also wrote a couple of `makefile`s to make fetching and installing the sources for the `pagedown` and `wmd` `underlaydir`s simpler.  And updated `doc/plugins/wmd.mdwn` to reflect these changes.