0.2: Dice All the Way Down

• 🎲 Add type for constants.
  Brentley Jones #48
• 🎲 Add convenience functions and operators for making dice and expressions.
  Jonathan Hoffman #44
• 🎲 Add the ability to determine the probability mass of an expression.
  Brentley Jones #42
• ⛔ Limit iOS deployment target to iOS 8.
  Brentley Jones #83
• 🔍 Add CustomStringLiteral and CustomDebugStringLiteral conformance to base types.
  Jonathan Hoffman #55
• 🎲 Add support for subtraction of expressions (e.g. d20 - 5)
  Jonathan Hoffman #80
• 🎲 Add min/max operations.
  Logan Johnson #64