Add datasets module with example datasets.

This PR adds the datasets module, which contains example datasets. These should decrease the learning curve for gmaps: the user can now just do `gmaps.datasets.load_dataset("earthquakes")` to have access to data he can plot on a map.

* Adds two datasets (taxi rides and earthquakes)
* Adds a "metadata.yaml" file to keep the metadata related to the datasets.
* Adds a `gmaps.datasets` module to load datasets.
* Fixes heat map example notebook to use datasets.
* Adds example notebook for weighted heat maps.

Fixes #26.