Add ability to add external overlay tile layers (fixes #1203)

![selection_469](https://cloud.githubusercontent.com/assets/546692/4772312/47765ca8-5b97-11e4-8686-879cacb1ae63.png)

I keep repeating that Geotrek should never be QGis :) But this feature is cool because you can add any overlay tile layer, static built with Tilemill or rendered from PostGIS with QGisMapserver or TileStache...