Projection changes for Proj4Leaflet

Some changes related to getting Proj4Leaflet to work with the new projection system.

Mostly minor details.

I changed ```CRS.getBounds``` into a ```bounds``` property, that will default to the bounds of the earth. The rationale is that even if the projection defines other bounds, it is rare to want to wrap latlngs according to those bounds. For special cases, like projections where coordinates aren't really lat/lng but still need wrap at other bounds, the `` `CRS.bounds``` property can simply be overridden.

In particular, a local projection like SWEREF99 has very small latlng bounds, and since wrapping is enabled by default, this would mean Leaflet would by default wrap the world between Finland and Denmark.

This also make arbitrary projections work even if they don't define bounds.