Export PDF's using jsPDF

As promised, here is an example on using the jsPDF library to creating PDF's from an `ol.Map`.

Each commit adds some additional feature to the export:

* The first only adds an example that exports a PDF, similar to the existing PNG example;
* The second commit gives the user the options to select an output size and resolution;
* As increasing the map size and/or resolution requires many more tiles, the third commit ensures these tiles have been loaded before exporting the map;
* The fourth & fifth commit simply restore the example so another export can be made.

Good point. But it might be a better idea to -for example- let the map dispatch the progress itself.
I'd rather wait for some feedback on the example as a whole before fixing this.