Added zoomTune option and continuous zoom

- zoomTune option allows to remap a level of zoom to a different
level of tiles, for instance to have more zoom levels than
available levels ot tiles.

- continous zoom allows to show the tile layer with a non
integer zoom level
Corrected error reported by Opera (without touchscreen)
Corrected error occuring with touchscreen and non-integer zoom
Force the box to have the same ratio than the map
Allow non integer zoom for touchscreen and scrollwheel
Modified getBoundsZoom() to compute the exact, non integer, zoom (in order to get accurate dragbox zoom)
Replaced shift step of 3 by 0.1 for testing purpose (continuous zoom)