WIP.
Added Exercise 2.7
Add Exercise 3.15
Add comments to Ex3.15.py detailing problem.
Merge branch 'fitze-master'
Merge branch 'oliverlee-kane'
Add Exercises from Chapters 2 and 3.
Add Exercise 6.13.
Add example of computing central inertia of uniform density torus.
Sliding Ladder Exampled

An example problem which solves for the equations of motion of a sliding
ladder in a frictionless world.
Added the controlled n pendulum example.
Updated the Readme.
Made readme markdown.
Wrapping and Figure

Wrapped the README file and added a figure for the problem.
Merge pull request #7 from v9joshi/Falling-Ladder-in-a-Frictionless-World

Sliding Ladder Example
Add more comments and references.
Merge pull request #4 from PythonDynamics/inertia_of_torus

Add example of computing central inertia of uniform density torus.
Added a mass spring system.
This system is used to analyze the working of webGL's inline IPython notebooks,using Three.js, and  TrackBallControls.js
Merge pull request #8 from PythonDynamics/npendulum

Added the controlled n pendulum example.
Fix typo in Exercise 6.13.