rabbitmq-c formula enhancements.

Update rabbitmq-c to latest stable version (v0.5.1).

Additionally improve the formula:
- 'rabbitmq' is not a required dependency.
- Codegen directory isn't required to build.
- Use CMake to build the package.
- Disable building of test and examples.
- Conditionally enable building command line tools, add popt dependency.

I'm a ruby n00b, please feel free to give pointers on style and correctness.