Chore/migrate to mobile element

[aaron-goff]

Related Issue

closes #146

Description

• Create methods that accept MobileElement in signature
• Deprecate methods that accept WebElement in signature
• Change return type of methods that were WebElement to MobileElement
• Modify tests to use MobileElement instead of WebElement
• Add some methods that were not created yet (but in line with our strategy of signatures of String, By, MobileElement)
• Add platform type boolean

Checklist

• [x] Have you added an explanation of what your changes do?
• [x] Have you written new tests for your core changes, as applicable?
• [x] Docs have been added / updated (for bug fixes / features)