[Nodeshot] Preview of the Elevation Profile Tool

[Nemesis]

Here's a preview of the elevation profile tool I'm going to add soon:
https://www.dropbox.com/s/0n50j7b16vz1wng/nodeshot-elevation-profile-dev.png?dl=0

The elevation data is taken via the Google Elevation API

 I tried using some other openstreetmap friendly service, but I could
not convert easily the data to GeoJSON and they all require an API key,
while the Google Elevation API allows quite many http requests without
an API key.

Not having to register an API key means the elevation profile tool will
work out of the box after installing nodeshot, which is nice (extra
steps are always annoying, look at the social login with facebook, is
very annoying having to setup all that shit each time).

The Google Elevation API is implemented through a proxy, the code is here:
https://github.com/ninuxorg/nodeshot/commit/b67ba142e9d0792d84c4bc62096594cbeadadcdc

The docs are here:
http://nodeshot.readthedocs.org/en/latest/topics/nodes.html#nodeshot-google-elevation-api-key

I plan to extract the Google Elevation GeoJSON Proxy feature into a
separate python package in a later phase.

If anybody won't like the Google API in the future, it should be fairly
easy to replace it with a different API, provided that the API returns a
GeoJSON with the third dimension (elevation).

Federico