Rewrote heatmap_view as a JS module.

This means it can now be installed as a nbextension. By putting the call
to load Google maps in there as well, we remove the init statement
completely. However, this requires the user running

%%javascript

IPython.load_extensions("js/heatmap_view")

which is not ideal, obviously. But I think this can be re-written as a
line magic.
Added JS loading code to loader.py

This is still a bit of a hack: gmaps statements cannot be called in the
same cell as the import statement.
Tidied loader.py and added information about hack.
Automatic installation of JS libraries.

This commit modifies the setup.py script to automatically install the JS
libraries.
Updated nbextensions directory name.

The setup script now installs JS extensions under nbextensions/gmaps_js,
rather than nbextensions/js.
Updated installation and "getting started" instructions.

Updated the installation instructions in the README and the CONTRIBUTING
guide to tell people to use "python setup.py
install/develop/install_data".

Updated the getting started instructions in the README to remove the
reference to the "init" function.
Merge pull request #5 from pbugnion/init_fix

Remove the need for a "gmaps.init()" function.