Messages support (in case of assertion failure)

[aseemk]

This patch adds an optional msg arg to all assertion methods:

foo.should.equal(bar, 'baz didn't have an effect!');
all.should.have.length(10, 'elements weren't added')

Because this wasn't possible for testing that properties exist:

// doesn't work, tests that foo.bar === 'custom message'
foo.should.have.property('bar', 'custom message')

I added a new verb called expose which is like property but doesn't take an optional value:

foo.should.expose('bar', 'custom message')

Hat tip to @dominictarr for pointing this out.

This isn't possible for non-method assertions like ok and arguments, which is why I'd love to change those to methods as well. Haven't figured out a clean way to do that without breaking existing code or having the change be opt-in via a config, which is kinda lame. What are your thoughts on that?

I also improved the static should.fail() --- which was just a wrapper for the native assert.fail() --- to actually set the passed-in message to the error's message property.

Other than that, some minor code and test cleanup. Let me know if something isn't clear.

Cheers, Aseem

[aseemk]

Funny, for some reason it includes all my old commits for should.exist() even though you already pulled those in. Any idea why that happens?

[aseemk]

Note that this pull request includes all of the changes in pull request #14. That now includes the automatic error message support:

someAsyncTask(function (err, result) {
    should.not.exist(err);    // if this fails, uses err as the message
    // ...
});