Added xpath and capybara selector options support

I have added support for xpath and other capybara selector options. In short element(s) can be defined like these.

element :some_element, :xpath, "//div[@class='a']"
element :another, "a.b", {:visible => true}

This was desirable in the project I am currently working on. I am sure, others can also benefit from this. I will appreciate your input on these updates. Thanks.