Plusminus 0.1.1

- Added useful customization methods to public API
  - safe_pow
  - safe_str_mult
  - constrained_factorial
  - ArithmeticParseException

- Modified example parsers to use these safe methods

- Moved many functions out of the base ArithmeticParser class and into
  BasicArithmeticParser, so that applications can choose to extend
  from the base class if trigonometric, etc. functions are not
  relevant to their particular domain.

- Moved example parsers into plusminus import tree, so that they can be
  imported and used:

      from plusminus.examples.example_parsers import DiceRollParser

      parser = DiceRollParser()
      print(parser.evaluate("3d20")

- Moved body of usage() out of bottle_repl and into the parser class
  itself so that repls can get basic usage text directly from the parser.