Retina support for WMS servers which don't like 512x512 tiles

Hello. This pull-request is the following [a previous one](https://github.com/Leaflet/Leaflet/pull/2535), but I don't know how to update it. So here it is.

I added a 'disableBigRetinaTiles' option and the tests are OK on my computer this time :D

Feel free to refuse it or to implement it in a totally different other way :-)