Remove calls to deprecated NSImage methods

Hi, the new preview of XCode 6 shows deprecation warnings for certain NSImage methods that this pull request corrects.

This first one was a call to -setScalesWhenResized:, which is documented [here](https://developer.apple.com/library/mac/documentation/Cocoa/Reference/ApplicationKit/Classes/NSImage_Class/DeprecationAppendix/AppendixADeprecatedAPI.html#//apple_ref/doc/uid/20000344-setScalesWhenResized_) to no longer have any actual effect since 10.6.

The second is to remove calls to -setFlipped: in the Aqua tab style. The documentation recommends using different methods to achieve the same effect. This patch uses a new private method on MMAquaTabStyle to redraw the images as flipped using [NSImage -lockFocusFlipped:YES], which is one of the documentation's recommended uses.

Please take a look at the commented out -flipImage: calls in the patch. In the original source, the images aquaTabBgDownGraphite and aquaTabBgDownNonKey were both not actually flipped – instead, aquaTabBgDown was flipped 3 times redundantly. I'm not sure what the intended actual behavior is, so this patch doesn't flip those two images which should mean that this style will look exactly the same. If the intended behavior was to flip those two images all along, the _flipImage: calls should be uncommented.