Device-width scaling fonts

• New `UIFont+CardsAgainst.swift` extension provides `blackCardFont`, `whiteCardFont`, and `voteButtonFont` getters that scale the original “base” font sizes (35, 20, and 17 respectively) according to the screen's native retina-irrelevant portrait width.
	» The idea behind this is that if the fonts scale based on screen width then the wrapping will be identical on all devices; bigger devices just get bigger text.  This is a game, after all, not a spreadsheet.
	» iPhone 6 was used as the base screen (since it's in the middle of the possible iPhone screen widths of 320pt, 375pt, and 414pt).  On iPhone 4/5s the font will be a bit smaller, and the iPhone 6+ a bit bigger, and on iPads— really big.  That said, the size relationships between pieces of text looks good, and the layout remains roughly the same regardless of screen size, so ultimately usability remains high and the layout isn't strange on some devices.
• Updated GameViewController's `blackCardLabel.font` assignments to use `UIFont.blackCardFont`.
	• `generateBlackCards()`'s in-loop `blackCardLabel.font` assignment changed to set the font after the `.attributedText`— otherwise we end up with the remote device's font size for their card.
• Updated GameViewController's `voteButton.titleLabel` to use `UIFont.voteButtonFont`.
• Updated WhiteCardCell's `label` to use `UIFont.whiteCardFont`.