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).
I understand the appeal but I think this is generally discouraged. The build process just generates so many extra files. I create a directory `opensim-core/` that contains `opensim-core` (the git repo) and `build`, and sometimes `debug` and `release` if I have a debug and a release build.