Python3 support and general improvements

• Python 3 compatibility.
• The “network.nearest_pois()” method can now return the labels of the pois rather than just the distances
• OSM data loading is now done via the osmnet package.
• Changes to support multiple graphs.
• Added reindex functions.
• Updated documentation.
• Switched code style checker in Travis CI to “pycodestyle”, which has replaced the “pep8” package.