Add directory path by using directory browser

With this you can add the path with directory browser, but seems there's an issue. I have to remove the first `//` on the layout path for make the addon works, otherwise you'll get the following error
```
Traceback (most recent call last):
  File "/home/aditia/.config/blender/2.72/scripts/addons/layout_tools.py", line 511, in invoke
    os.makedirs(layoutdir)
  File "/home/aditia/blender-git/build_linux/bin/2.72/python/lib/python3.4/os.py", line 227, in makedirs
    makedirs(head, mode, exist_ok)
  File "/home/aditia/blender-git/build_linux/bin/2.72/python/lib/python3.4/os.py", line 237, in makedirs
    mkdir(name, mode)
PermissionError: [Errno 13] Permission denied: '//layout_files'
```