Include icons in xpi base path (fixes #197)

This fixes icons not showing when they were not placed as icon.png in the base directory (fixing #197). I tried to do the same thing cfx does, automatically including the icon file in the xpi root.

Also some tests! :tada: 

Even though I do it here, I'm of the general opinion that we should be careful with creating more temporary files and cleaning them up after the xpi is created. This can only be done to a certain extent, as it comes with a lot of overhead (checking if the file exists, testing that we do not overwrite/delete the file) that could be avoided by using a tmp location.