Use monospace digits for time label

Fixes #351 (closed).

I opted to preserve the font that is set in the xib, but add the monospaced numbers OpenType attribute. If you prefer, I can set the font directly in code, which is the much simpler one-liner:

self.timeDisplayButton.titleLabel.font = [UIFont monospacedDigitSystemFontOfSize:15 weight:UIFontWeightBold];

However, it seemed like most font configuration in the app is done in xibs rather than in code. But I'm happy to do whichever style people prefer.