error when in directories w/o package.json & CHANGELOG.md

[ungoldman]

e.g.

∴ ng@crimelab ~/dev/github $ node ./gh-release/bin/cli.js

module.js:340
    throw err;
          ^
Error: Cannot find module '/Users/ng/dev/github/package.json'
    at Function.Module._resolveFilename (module.js:338:15)
    at Function.Module._load (module.js:280:25)
    at Module.require (module.js:364:17)
    at require (module.js:380:17)
    at getDefaults (/Users/ng/dev/github/gh-release/lib/get-defaults.js:6:13)
    at /Users/ng/dev/github/gh-release/bin/cli.js:68:3
    at auth (/Users/ng/dev/github/gh-release/node_modules/ghauth/ghauth.js:140:14)
    at Object.<anonymous> (/Users/ng/dev/github/gh-release/bin/cli.js:65:1)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)

getDefaults() shouldn't be called unless gh-release is being run in a valid directory. if the directory is invalid, gh-release should return an error message and/or usage info.