Add getters and setters for TileUrlFunction and TileLoadFunction properties

This PR add getters and setters for `TileUrlFunction` and `TileLoadFunction` properties in `ol.source.TileImage`.

These functions are needed  to use tiles in offline mode, see https://github.com/oterral/ol3/commit/87399ef85a206a92e752723732ca2bd339762289 for an example of use.