Added OS X starter kit referenced in the docs

This is referenced in the docs but doesn't exist in `tumblr/gocircuit`.

I generated this by building zookeeper 3.4.5 as suggested by the docs and copying:

* `{zk}/src/c/include/*` into `{circuit}/misc/starter-kit-osx/zookeeper/include`
* `{zk}/src/c/generated/*` into `{circuit}/misc/starter-kit-osx/zookeeper/include`
* `{zk}/src/c/.libs/libzookeeper_*.a` into `{circuit}/misc/starter-kit-osx/zookeeper/include`

System details:

`Darwin burke.local 12.2.0 Darwin Kernel Version 12.2.0: Sat Aug 25 00:48:52 PDT 2012; root:xnu-2050.18.24~1/RELEASE_X86_64 x86_64`