Add a style option for vector layers

This can be a single ol.style.Style, and array of styles, or a style function.
Add tests for vector layer
Use style option in vector layer examples

Providing a styleFunction option still works because the layer property is still named styleFunction.
Give vector image source a style option
Give feature overlays a style option
Common code for creating a style function
Delete style option before passing to base
Give the draw interaction a style option