wix: drop bin/ folder, add root install folder to %PATH%
There's been a lot of blow back about the batch files and bin folder, the most
serious of which is that batch files break other batch files so for Mercurial
1.8.1 we'll go back to placing the root folder in the %PATH%.
My hope is that the DLL conflicts we saw in the past were mostly caused by GTK.
We'll see which way inflicts more pain.
Refs #60, #201, #239 and Mercurial issues 2658 and 2673