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.
I think we can add a separate ticket to add this capability rather than adding it within this pull request if everyone feels this would be useful.  I definitely see value for existing projects.