First commit. Added description and constants to pendulum.py.

pendulum.py

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+"""
+    Represents an inverted pendulum, keeping track of angle, angular velocity and angular acceleration.
+    
+    angular acceleration = (g*sin(x) - a*m*l*(v^2)*sin(2*x)/2 - a*cos(x)*u)/(4*l/3 - a*m*l*cos^2(x))
+    x = angle
+    v = angular velocity
+    g = acceleration due to gravity
+    m = mass of the pendulum
+    M = mass of the cart
+    l = length of the pendulum
+    a = 1/(m+M)
+"""
+
+# Constants
+g = 9.8
+m = 2.0
+M = 8.0
+l = 0.5
+a = 1.0/(m+M)