Add the getElementName method

Because this method is used in all `Element` classes to compute the tag name, this is then easier to modify how the tag name is computed, for instance it is easier to remove the clark notation by extending an `Element` class.

Used to simplify maintenance of https://github.com/fruux/sabre-vobject/pull/160.
It explains the role of this method, why it shouldn't be part of the interface API documentation :-)?