Tweaked the installer

I've tweaked the script to check if the installation directory already has PwnSTAR. 
If the folder exists then the user is given an option to enter a new installation directory. 
Default installation directory is /opt 

Sample Screen Shot: 
![tweak](https://cloud.githubusercontent.com/assets/4489002/4356286/9f3fe714-4257-11e4-8e1c-72e614083a21.png)