Add richer explanations for falsified properties

[wuest]

Closes #4

Failures are now given rich output to help developers understand the conditions under which the properties fail to hold. Examples from the fails Rake job:

  1) Failure:
PropertyTest#test_also_falsifiable [test/should_fail/property_fails.rb:15]:
A counterexample to a property has been found after 16 valid examples.
The values at the time of the failure were:
        xs: []
        ys: []

  2) Failure:
PropertyTest#test_falsifiable [test/should_fail/property_fails.rb:7]:
A counterexample to a property has been found after 0 valid examples.
The values at the time of the failure were:
        n: 0
        m: -1