Merge pull request #99 from jesstelford/feature/element-exist

Add `.waitForElementExist(selector, timeout)` method
add documentation for waitForElementExist
1.10.0
Handle 'stale element' as element not existing

Occasionally webdriver throws an error about the element reference being stale.
Let's handle that case as the element doesn't yet exist. All other errors are re
thrown as before.