More flexible install.sh

The installer (install.sh) expects the folder "vimified" located in $HOME. If the installer is called from another location, then the symlinks do not work.

This patch defines a environment variable INSTALLDIR, that contains the path of the current folder ($PWD). Now the soft links reference the absolute path to the "vimified" folder.

Feel free to skip this patch and implement these absolute pathes otherways. But please make the installer more robust.