Descriptive README, addresses #108

[chrisdembia]

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).
• 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.

[chrisdembia]

Also, the dependency information may be wrong for Windows and Mac OS X. I didn't try to check; I took that info from doc/INSTALL.txt

Another thing: hopefully @sherm1 can refine the list of features.

[sherm1]

Great start -- thanks. The animated gif is a nice touch! Is that really 11000 bodies?

I'll merge this and then we can tweak it more as time permits. Much appreciated, Chris.

[chrisdembia]

Great. Thanks. I got the figure from you in #49. It's a really cool video!

[sherm1]

I see -- those were actually two separate comments, ambiguously worded. I should have said: (1) Sam Flores holds the record with an 11000-body chain, and (2) He also makes cool movies like the Simbios-spelling RNAs.

I actually don't know how many bodies were used in that movie so I changed the text to say "thousands".