V2.0 - new test syntax and other improvements

[philc]

I'm calling this v2.0 because it breaks existing test syntax. Changes:

• Update the test syntax to use closures rather than arrays. This allows for more flexibility, is easier for code editors to indent, and matches the syntax used by most JS testing libraries.
• In test failure output, print complex objects on separate lines to improve readability.
• Add "should.only" and "context.only" for programmatically limiting which tests will be run.