Update README.md 

* Put in a gif of a running model
* Put in some sample C++ code and the resulting animation
* Put in build instructions for Windows, Mac w/Xcode, and Ubuntu

I know we haven't discussed what *should* go into the README, so it's fine if we don't like this at all. 

I think it's understandable if people think it doesn't make sense to have sample code in the README. It's what @tkuchida and I did for Simbody's README, but it may not make sense for OpenSim's users. Also, the sample code is fairly long, even though I tried to make it as short as I could.

Also, people may think that the build instructions should remain at http://simtk-confluence.stanford.edu:8080/pages/viewpage.action?pageId=6750730. I think the build instructions belong in the repository, since build instructions can vary from commit to commit. Also, @tkuchida tried to use that website to build off master and had issues b/c those build instructions are somewhat tailored to released versions.

I'm happy to discuss what should go in the README @jenhicks @jimmyDunne .

It'd be nice if @apoorvar could read over the Xcode instructions to see if I messed anything up. I know @sherm1 said it doesn't make sense to give Xcode instructions for Simbody, but I thought OpenSim users will be less likely to want to use the command line on a Mac (thinking of this year's 485 students).
Will fix this. What do you mean by addressing this more systematically? In this README, or in some confluence page, or in example code?