records: prettify MarcXML on creation

* Makes MarcXML more readable and properly indented.

* Turns [this](http://pastebin.com/cYRE2s68) into [this](http://pastebin.com/eqZ92A47).

See #1926.
Hmm not sure if it's useful... But since I'm *forcing* it to have 4 spaces of indentation, why not make it customizable? I'll ammend it, thanks for the suggestion! ;)