Fixed bug avoiding drawing curve

If the variable holding the canvas layer is not called layer (as in the
demo file), no curve would be drawn at all.
New addTo function (standard in Leaflet)

Additionally to the addLayerTo function, an addTo function has been
added. The addTo function is the standard function in other Leaflet
layers for adding the layer to the map.
Added option to deactivate point filter

Points to be drawn are automatically filtered using the QuadTree. For
drawing curves, simple filtering by points does not work, hence the
option to deactivate this filtering. This is certainly not an elegant
solution, but it is probably still ok for many situations. Need to
introduce bounds checking based on geometry bounding boxes or similar
(again a simple check won’t work for some curves).
Added possibility to change curve colour and width

The stroke colour and line width of each curve can now be defined
separately in the data.
Added dynamic filtering of curves

Curves can now be filtered dynamically by providing a function within
the data dictionary. If the function returns true the curve is drawn
and otherwise it is skipped. If no draw function is provided, all
curves are drawn.
Added redraw function

redraw function is known from other Leaflet layers, such as the
TileLayer.Canvas for example.
Replaced curve styling with style function

Instead of providing individual stroke style and line width for each
curve, it is now possible to provide a style function that returns a
dictionary with the stroke style and line width. The style function
receives the data for the corresponding the line, enabling dynamic
rendering of the curve style.
Merge pull request #1 from christiankaiser/master

Fixed bug avoiding drawing curve