Added option whether or not to use private APIs. If BS_USE_PRIVATE_API is set to 0, it will create an NSToolbar with a dummy view to stretch the titlebar. The limitation with this is that the height cannot be below 41. It should be OK for the Mac App Store, though.

If BS_USE_PRIVATE_API is set to 1, it will swizzle the NSThemeFrame's methods to modify the titlebar height directly. The limitation with this is that it uses private APIs, so it can't be used in the App Store.
Updated readme file