markers directive support for expressions

@aesnyder @gitnik @kandizzy @cthrax @zacronos 

So this might cause a movement to 2.1.0 as this is a breaking change to how Markers handles functions. IE it will no longer call functions within a model. Instead it will call expressions which expect a model as a argument. At the same time it evaluates objects, and function al string expressions in html as well. 

If it the expression results in a string then it falls to the models properties. This functionality is already there for polyline, polylines polygon, and polygon(s)
mutability of options.position updates the internal position. It might be easier just to point all properties to opts['whatever'] ... todo.