Change the application data directory on unix to .MultiBit

Change the application data directory on unix to .MultiBit instead of MulltiBit, the dot matter,
it will make the directory hidden.
Preserve directory if was already created without the dot, just to avoid the user have to reimport walltes or so (i can do something about it later).


also tried to change the default directory path to /opt but it will only work if is executed as root, so i need research this a little bit more