All element assertions that find an element return it

Adds, documents and tests useful return values to:

* `browser.assert.elementIsVisible(selector)`
* `browser.assert.elementNotVisible(selector)`
* `browser.assert.elementExists(selector)`

...so they work just like all the other `browser.assert.element*` methods (save `browser.assert.elementDoesntExist(selector)` – which by definition can't).

Also added tests to assert the old API methods return the element, and corrected the name of one of same tests.