Please respect DESTDIR

Hoi Joren,

I'm trying to package Olaf for my distro. It would be great if I could do `make DESTDIR="$pkgdir/" install`.

Having sudo in an install script is certainly not liked by Gentoo or Arch folk:
```
	mkdir -p ~/.olaf/db/
	sudo mv bin/olaf_c /usr/local/bin/olaf_c
	sudo chmod +x /usr/local/bin/olaf_c
	sudo cp olaf.rb /usr/local/bin/olaf
	sudo chmod +x /usr/local/bin/olaf
```