Add isRequired() and isNotRequired() assertions

[Turbo87]

This assertion should check if the required property of e.g. an <input> element is true or not.

Example:

assert.dom('.username-field').isRequired();
assert.dom('.coupon-code').isNotRequired();

[scalvert]

Also, the attribute being present also represents true, so the assertion should take that into consideration too.

[billybonks]

@Turbo87 guess you can mark this as complete