test.html: use google hosted jquery, not local

We don't bundle a copy of jquery, so modify the example page to
instead load a public copy.
Add Selectable typeclass, replace select* functions
rename createJQuery to selectInContext, generalize type

The second (optional) argument to jQuery() specifies a selector
context (eg. DOM Element) to restrict the search within. By default
this is the doucment root.

This provides a more useful name for this use of the function, and
generalizes its type so that any Selectable can be used for either
argument.