Added support for missing cameras

Cameras are no longer added if missing
Cameras submenu will scroll to the first available camera
    or the Back button if none are available
Camera button names were changed to make this easier/cleaner
fix comments from pull request

pep8
combined 2 if statements
use list comprehension for get_btn() function
added a space to fix indentation
Merge pull request #1 from RethinkRobotics/camera_fix

Added support for missing cameras