An improved way to adjust the toolbar item size.

- simply set toolbarItemSize property on MASPreferencesWindowController.
- MASPreferencesWindowController now uses a styled NSButton as a custom view in all cases.
lolib
Mention default value for toolbarItemSize
Now using a self referencing pointer for the associated object key.
Merge pull request #2 from github/custom-toolbar-size

An improved way to adjust the toolbar item size