Simple arithmetic evaluator for Ruby

Based on http://rosettacode.org/wiki/Arithmetic_evaluation#Ruby but adds:

• tests
• support for nicer errors on invalid expressions
• optional spaces between operators and operands
• uses BigDecimal instead of floats

Shortcomings:

• only supports + - * / operators
• no localization

expression = Arithmetic.parse("-2 * (1+3.5)")
expression.eval # => -18
expression.to_s # => "-2 * (1 + 3.5)"

Arithmetic.parse("2 + wtf?") # => raises Arithmetic::InvalidExpression with the 
                             # original expression as the message

Please ensure that decimal separators are decimals ('.'). Use a gem like Delocalize to perform this conversion.