Make error messages nice

[yamadapc]

When a doctest fails, it's completely unacceptable that the error be something like "Assertion failed: false === true"; we can very easily import should.js to give diffs and fancy expectations, but there's more to this.

When a test-case fails, we need to provide not only why an example failed, but the example's code as parsed and its context documented function.

This is also related to #3, since compiling to a mocha spec we wouldn't have to write too much code around this.

[yamadapc]

This is no longer relevant, since we compile to use should and support mocha.