[vector-api] Add ol.format.GPX

This PR adds GPX support.

It also adds a `styleFunction` option to `ol.interaction.DragAndDrop` which allows the drag-and-drop example to be extended to many more formats. This was done in this PR to facilitate testing of `ol.source.GPX`, as GPX files do not contain style information themselves.
![ol3-gpx](https://f.cloud.github.com/assets/6942/1933548/4d18c48a-7edd-11e3-8558-31dade2ad764.png)