Adding assert for type and tag

Implements `assertWrappedElement` to check for correct wrapped element in typified elements.

Closes #53 
If we move this line down (after the `$this->_wrappedElement` is assigned), then we can simplify checking code like so:

1. don't pass `$wrapped_element` to any of assertion methods
2. use `$this->getTagName()` instead of `$wrapped_element->getTagName()`
3. use `$this->getAttribute(...)` instead of `$wrapped_element->getAttribute(...)`

No changes in test code are required because essentially same `WebElement` class mock is used in new code version as well.