Make sure Object.keys arg is an object

I was getting the following when calling `configStore()` with no arg, which seems like it should be valid to do:
```
/Users/mikel/Projects/openbadges-badgekit/node_modules/config-store/configStore.js:118
    Object.keys(config).forEach(function (key) {
           ^
TypeError: Object.keys called on non-object
    at Function.keys (native)
    at parse (/Users/mikel/Projects/openbadges-badgekit/node_modules/config-store/configStore.js:118:12)
    at configStore (/Users/mikel/Projects/openbadges-badgekit/node_modules/config-store/configStore.js:63:16)
    at Object.<anonymous> (/Users/mikel/Projects/openbadges-badgekit/app/lib/config.js:4:18)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Module.require (module.js:364:17)
    at require (module.js:380:17)
```