Some slight changes to prepare.sh

Slight changes to prepare.sh , pass a param for selected os. exit if passed param value isn't found in array.  Some changes to mkdir , adding -p switches etc.

Takes a param in terminal e.g  $ prepare.sh ubuntu12.04 
mkdir -p , 
chmod more folders in 1 line

Not huge changes , but make it easier for installing over terminal. Only have this for unix