Add ability to attach a spotlight to an agent

Calling Model.setSpotlight(agent) will add a "spotlight"
to an agent, similar to the NetLogo "watch" function.
This allows a user to easily follow a single agent.

The spotlight is removed when Model.removeSpotlight()
is called, or when the agent dies.

There is also a shortcut for Model.setSpotlight(breed),
which will pick a random member of a breed.

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This can be seen in our current Global Climate model, available here: http://concord-consortium.github.com/agentscript/models/global-climate.html

Try clicking one of the "Follow" buttons on the bottom right.

Testing has found that this does not appear to affect the model's performance at all.

![AgentScript Model](https://f.cloud.github.com/assets/35721/299915/59e4b21c-9588-11e2-8817-430613506992.jpg)