Switch to cson-safe for parsing

As discussed in [an issue on a different project](https://github.com/bevry/cson/issues/33), using eval for config files is a suboptimal solution. What I'm suggesting here is an approach based on walking the CoffeeScript AST instead. The parser allows for statically evaluated arithmetic expressions (e.g. `myFlag: 1 << 3`). If that's a concern, I'd be happy to make it optional. I chose to implement the parsing in its own module since I think that having a minimalistic parsing library that just offers the `JSON.{parse,stringify}` interface could be valuable to people.

More information about the behavior of the parser:

* [parse test in cson-safe](https://github.com/groupon/cson-safe/blob/master/test/parse.coffee)
* [Error message](https://github.com/groupon/cson-safe/blob/37912fc799990c839b5e686013ed7976915c902d/test/parse.coffee#L82)

See also: bevry/cson#34