Add assert.equal and assert.strictEqual

[lee-houghton]

I like this library, it makes writing assertions much DRYer! But I did miss being able to assert that two things are equal and having the error message include the values of the things that were different, rather than just the names. I added an assert.equal and assert.strictEqual, and thought it might be useful to make a pull request...

I've extracted the source code extraction into a fail function, which takes an optional extraMessage parameter to give a bit more information about the error. I've only implemented assert.equal and assert.strictEqual, though I'm sure something like assert.deepEqual would be useful too. I've used util.inspect for stringifying the values.