Cache storage, scheme and mbutil

Some changes:
* add possibility to store cache using WMTS scheme
* tell mbutil if the tiles are stored at WMTS standart (so it will only be flipped in mbutil, as it internally use TMS)
* add a 'scheme' attribute in Cache 'tms' as value by default (useful when reading a 'Dummy' cache from an MBTiles to create a new one, for example), and modifiable in a 'Disk' cache only.

Do you think it is OK? If so, I will add a few lines in the README to explain it.