Add isDisabled() assertion

[livet01]

Adding checked and disabled states to be able to assert them easily:

assert.ok(find('input[type="checkbox"]').checked);
assert.dom('input[type="checkbox"]').isChecked();

assert.ok(find('input[type="checkbox"]').disabled); 
assert.dom('input[type="checkbox"]').isDisabled();

Also what about readonly or required?

[Turbo87]

I guess we should probably have a generic hasProperty() assertion roughly similar to hasAttribute(), but having those shortcut assertions for the most common properties should also be useful.