Factory functions for creating layers

[pbugnion]

Creating layers is currently brittle and error prone. Since we use traitlets, the argument type is fixed. This breaks duck typing and is generally surprising to Python developers. For instance, at the moment, the data argument for a heatmap only takes a list of tuples. It should really take any iterable. The solution is to create factory functions for the layers that let us massage the arguments before the create the widget model itself.

This will address several issues:

• the ugly interface to the Directions widget: instead of passing the start and end point and waypoints as an array, we can just pass them as a arguments to the functions, with a signature like directions_layer(start, end, waypoints=None, **kwargs).
• the ugly way in which WeightedHeatmaps are implemented. Instead of having two different ways of creating the heat maps, we can just have a single heatmap_layer(locations, weights=None). This will create either a WeightedHeatmap or a Heatmap internally.

The markers and symbols layer already use a factory method. This PR will just bring the rest of the layers in line.

This PR will fix #65, #66 and #68.

 Remaining tasks:

• factory function for heatmaps
• factory function for directions
• refactor datasets implementation to integrate better with factory functions
• docstrings for factory functions
• rewrite tutorial to make use of factory functions
• add unit tests for marker and symbol layers to verify that they work with common data types

[pbugnion]

The interface for heat maps and weighted heat maps is now unified:

heatmap = gmaps.heatmap_layer(locations, weights=None)

We can now do this: