implements .rockspec file extension on windows

See screenie; 
![image](https://f.cloud.github.com/assets/1077043/356672/d8fde54e-a110-11e2-87fd-508951862de8.png)

Registry can be written using `/R` installer commandline option. It also adds an icon (from LuaForWindows)

besides that the installer option `/P` is now REQUIRED, so simply typing `install` will not do the ugly default of `c:\luarocks\` but will show an error and hint.