Handle object property starting with `@`

[SimenB]

We have a logging contract which says timestamp should be called @timstamp and version should be @version. This module throws when it tries to serialise it (on node@6 at least).

Example:

require('fast-json-stringify')({
    title: 'Logging Schema',
    type: 'object',
    properties: {
        '@version': {
            type: 'integer',
        },
    },
});

undefined:61
      if (obj.@version !== undefined) {
              ^
SyntaxError: Invalid or unexpected token
    at build (/Users/simbekkh/repos/node-fiaas-logger/node_modules/fast-json-stringify/index.js:80:11)
    at Object.<anonymous> (/Users/simbekkh/repos/node-fiaas-logger/sup.js:5:19)
    at Module._compile (module.js:570:32)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)
    at Function.Module._load (module.js:438:3)
    at Module.runMain (module.js:604:10)
    at run (bootstrap_node.js:393:7)
    at startup (bootstrap_node.js:150:9)