Moving the feature management to the layer

The source will be responsible for fetching vector data.
Vector related exports
zoomFactor defaults to 2
View2D zoom may animate pan
MouseWheelZoom interaction animates zoom
Debounce mousewheel zoom events
Change View2D function names

Also make the TouchZoom interaction no longer use a private View2D function.
Add a rotateNoConstraint View2D method
Make TouchRotate interaction use rotateNoConstraint
Correct Stamen source
Add Stamen example
Add a snapToZero rotation constraint
Do not apply rotation constraint while rotating

This is on par to what the TouchRotate interaction does.
Make animating rotation possible
Animate rotation when releasing mouse or fingers
2DView refactoring

This refactoring makes animating zoom, as opposed to zoomByDelta, possible.
Animate zoom after pinch
Animate double click/tap zoom
Do not use tiledWMSOptions in closure function

See #269.
WMS GetMap request parameters shall be params

To avoid surprises, we configure everything that is a WMS GetMap
request parameter in the params object, and not as direct
configuration option. This affects the VERSION and TRANSPARENT
params.