Switch to cson-safe for parsing

This uses the approach I outlined in [a previous comment](https://github.com/bevry/cson/issues/33#issuecomment-36391887) for parsing. I removed the test that requires parsing of invalid/unsafe coffee-script as part of a CSON file and moved test `8` to be now test `7`. The new 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)