Add after/before DOM helper methods.

Right now appendChild and prependChild give a lot of flexibility in building out the Galleria DOM, but there are some things that are very difficult to do (rearrange just parts of the DOM, or inserting an element between others). Therefore I propose adding two more DOM manipulation wrappers:

* Galleria.after( siblingID, elementID );
* Galleria.before( siblingID, elementID );

Technically, this can currently be accomplished directly in jQuery with `Galleria.$("myId").after( Galleria.get("myId") )`,  but that's a bit verbose. Since all of your other DOM tools are nicely wrapped behind Galleria's API, it'd be great to make this work the same.