Performance improvements on Element.findElement()

Made multiple performance improvements:
- Allow users to specify locator type upon construction of a widget (XPath, ID, Name, etc) so that we can directly retrieve the By locator
- Store WebElement object for reuse
- Store By locator for reuse

This is a fix for #64.
Adding a 3rd:
3) Saving the WebElement so that it is only found once

With 3 ... starting to agree with @mike-glorioso . Can we roll back so that the only changes are locatorType?