Descriptive README, addresses #108

Addresses #108. Other relevant issues: #49, #50, #60.

* Add travis build status icon.
* Show cool GIF made by Sam Flores (will not show in this PR because I'm using links that'll only be valid once the PR is accepted. To see the GIF in action, see [this]( https://github.com/chrisdembia/simbody/tree/c79380b60cf2a77d16e5801cfa71447d4857f134)).
* Provide source code for double pendulum example.
* List a bunch of links for where people can find forums, instructions, etc.
* List project dependencies.
* List basic command-line build/install instructions.
* List other dynamics engines.

I'm overall dissatisfied with the build/install instructions I've put in the README. I realize our userbase is largely Windows users, but it doesn't feel possible to give Windows users anything too useful in the README. It seems that the README's usually are suited for people using a terminal.

Unfortunately, I don't have too much more time to work on this.