Added toHaveSelected matcher for determining if a jQuery selector selects some given elements

I was writing some tests and noticed we have toContain, which uses $.find under the hood, but this only checks for children in the selector, while I was trying to determine if a selector that selects many elements (via .add) selects a given element or not.

Hope I just needed to write the matcher and specs, let me know if anything looks out of place! And thank you for providing this great library for jasmine. :tada: :)