Added basic support for datasets.

Gmaps now contains a 'datasets' module that contains datasets, a little
like scikit-learn. The idea is that this will lessen the learning curve
somewhat.

Can now do:

    import gmaps
    gmaps.heatmap(gmaps.load_dataset('taxi_rides'))

Only contains SF taxi data at the moment.
Modified heatmap ipy2.3 example to use datasets.
Added earthquake dataset.

Added dataset of all recorded earthquakes from 12.11.2014 to 12.12.2014.
This is useful for demonstrating weighted heatmaps.
Added example IPython 2.3 notebook for weighted heatmaps.

Uses the earthquake dataset.
Amended README to use dataset in example.

The README file gives a very basic example. This example now uses one
of the datasets provided, rather than hard-coding one in.
Added IPython 3 versions of example notebooks.
Merge pull request #35 from pbugnion/datasets

Add datasets module with example datasets.