refs #427 - Added sendKeys options keepFocus (with docs and test)

Adds to ``sendKeys()`` an option to prevent losing focus which is useful when testing autocomplete widgets. For example, to pick the first suggestion in an autocomplete drop down, one could use:

```js
this.sendKeys('input[name="language"]', 'fr', {keepFocus: true});
this.click('#autocomplete li:first-child');
```

Also:

- The documentation has been updated. 
- The test harness has been updated to test that the option works with an autocomplete widget. A custom bare-bone autocomplete widget had been added to ``form.html`` to allow testing without impacting the speed of the test harness.