Allows passing options through has_button? to has_selector?

This enables assertions on the presence of disabled buttons by doing:

    assert has_button?('Button text', :disabled => true)

in a similar way to:

    find_field("Disabled Checkbox", :disabled => true)
Merge pull request #1139 from carols10cents/has_button_options

Allows passing options through has_button? to has_selector?