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.
Based on the idea of implementing the findElement method selection, I would recommend keeping Element(String locator) and updating the javadoc. It should not be deprecated in that case.