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.

this will have to be exportable if it's to be used in apps. There should also be some docs: what's it for? when/how do I use it?