Fix dynamic templating for Angular 1.2

Small fix which should take care of https://github.com/siddii/angular-timer/issues/29

If the timer tag has dynamic content (e.g., `<timer>my content with {{seconds}} etc</timer>`), the controller now treats it the same way as it does in the default case (`<timer/>`, which inserts `<span>{{millis}}</span>`): it grabs the desired markup from the element and `$compile`s it with the timer's isolated scope; otherwise in Angular 1.2 it would get rendered with the parent scope and have no access to the timer's scope vars.

This does wrap the dynamic content in `span` tags so that Angular can compile it properly. Maybe it would make more sense for them to be `div`s so that you could use almost any arbitrary markup within the `timer` tag.