Assertion ElementHasAttributes added.

Proposing, `browser.assert.elementHasAttributes( [docString,] selector, attributesHash)`
That asserts that the element contains `attribute:value` pairs specified by attributesHash.

You can omit this test, as javascript Objects only have String keys. (Indexing on anything else casts to string, so you can't have a `0` property and a different `"0"` one; only `"0"` exists on the object.)