panInsideBounds: fit or center, pass options, remove boundsMinZoom

Deltas are calculated on x, y pixel coordinates separately.
Options are propagated from setMaxBounds to panBy.
No panBy loops. Fixes #1908.
test Map#setMaxBounds and Map#panInsideBounds
Merge branch 'patch-1' of https://github.com/kapouer/Leaflet into maxbounds

* 'patch-1' of https://github.com/kapouer/Leaflet:
  test Map#setMaxBounds and Map#panInsideBounds
  panInsideBounds: fit or center, pass options, remove boundsMinZoom
add max bounds outline to debug file for easier testing
reimplement max bounds with proper zooming
proper rounding of max bounds offsets
adjust whitespace and add some comments
Merge pull request #2187 from Leaflet/maxbounds

Reimplement max bounds enforcement with proper zooming and lookahead