Multiple classes

Add AssertJ support for matching against one of many CSS classes on an element.

 `hasClass()` on `findFirst()` and `first()` had different semantics with regards to matching strings vs. substrings.  Now, they both follow the HTML/CSS rule of allowing space-separated names.  For instance, `findFirst(".alpha.beta").hasClass("alpha")` and `findFirst(".alpha.beta").hasClass("beta")` will both be true assertions.

We did not change fest assert because it is deprecated.

Paired on by me and @MarkyMarkMcDonald.

Fixes #120.