Allow title bar gradient colors to be configurable.

[jarod2d]

It would be useful to be able to set the gradient colors for the title bar view. I know you can set a drawing block and use custom colors with that, but if all you want to do is modify the gradient you're going to have to reimplement all of the other drawing code.

In my particular case this is useful because I have Safari-like tabs underneath my title bar. The default NSWindow title bar color transitions well into the tabs, but INAppStoreWindow's is a bit too dark.

[jarod2d]

I actually decided to add this functionality myself -- see issue #79 for the pull request which includes this feature.