Sample semantic navigation agent example

[ekolve]

Issue to track adding an example of training an agent to perform semantic navigation.

[ekolve]

Closing, class segmented and instance segmented images can now be retrieved by initializing the controller as follows:

event = controller.step('Initialize', renderClassImage=True, renderObjectImage=True)

Event has two new attributes: instance_segmentation_frame and class_segmentation_frame. The site (http://ai2thor.allenai.org) will be updated shortly with additional documentation about this.