Added Greedy Shortest Path Follower (#51)

.gitignore

baselines/__init__.py

examples/shortest_path_follower_example.py

examples/visualization_examples.py

habitat/core/simulator.py

habitat/sims/habitat_simulator.py

habitat/tasks/nav/nav_task.py

habitat/tasks/nav/shortest_path_follower.py

habitat/utils/__init__.py

habitat/utils/geometry_utils.py

habitat/utils/visualizations/__init__.py

habitat/utils/visualizations/maps.py

requirements.txt

test/test_habitat_example.py