Fix intermittent CALayerInvalidGeometry crash after memory warning

[johnboiles]

Fixes #44

I could reproduce this crash by mashing cmd-shift-m to trigger memory warnings around the time JDFlipNumberView is laying out. This crash was caused by _topImages getting cleared out upon memory warning. Here's the sequence of events:

1. Memory warning -- _topImages is cleared
2. JDFlipNumberView layoutSubviews is called
3. JDFlipNumberDigitView's sizeThatFits: method calls [self imageSize] which calls [factory imageSizeForBundleNamed:self.imageBundleName]
4. imageSizeForBundleNamed: doesn't recreate _topImages so the size is returned as CGSizeZero
5. JDFlipNumberDigitView's sizeThatFits: runs the line CGFloat origRatioW = imageSize.width/(imageSize.height*2);, resulting in a NaN (divide by 0)
6. The NaN is set as the height for the JDFlipNumberDigitView's frame
7. CALayerInvalidGeometry crash

This fixes the issue by using topImagesForBundleNamed: instead of accessing topImages directly. topImagesForBundleNamed: recreates the topImages if they're not available.

[calimarkus]

awesome thanks.