Add pydy.codegen and pydy.viz examples to double_pendulum.
Add explanation of viz calls.
Remove errant comment; specify lambdify.

Lambdify is the default generator, but I'm specifying it explicitly now.
Remove specifying lambdify.
Address @tarzzz' and @moorepants' feedback.

* Removed `specified` arguments to codegen.
* Fixed line wrapping of my comments.
* Put calls to `orientnew` and `locatenew` on their own lines.
Merge pull request #49 from chrisdembia/viz-example

Simple example of pydy.codegen and pydy.viz