Add markers to arena walls

test.py updated to test this.
Fix headings returned by see() method

(Current heading was not being converted to degrees before being
subtracted from the marker's rot_y value.)
Make headings wrap around at ±pi rads, change sign

Previously, a positive heading meant that the robot had turned anti-
clockwise from its starting position, which makes sense but is
inconsistent with the coordinate system (as positive Y is down instead
of up). A positive heading now indicates a clockwise turn.
Add a grabber to the virtual robot

The SimRobot.grab and SimRobot.release methods allow user code to grab
and release tokens, respectively. They return a boolean value
indicating success or failure. The robot can only hold one token at
once. A held token will move with the robot, and is coloured green.
Merge branch 'master' of github.com:Fodaro/sr-turtle

Conflicts:
	markers.py
	test.py