Fixed `this.$(selector)` returning cached result

This is a slightly cleaned up version of my proposal in #55 for consideration :smile:

* Deprecated `this.append` in favor of `this.render` as discussed.
* Made `this.$(...)` an alias to `this.subject().$(...)` but it also ensures that the subject has been rendered into the DOM.

This seems to match my expectations quite nicely – my test suite is passing with this patch.

* * * 

Existing usage of `this.$()` will continue to function the same way, while
`this.$(selector)` will work similarly to `this.subject().$(selector)` but
will first ensure that the subject has been rendered.

Also deprecated `this.append()` in favor of its new name, `this.render()` to
help better communicate its purpose.

Fixes #55.