Fix requireing module on windows

The following happens when I require the module on windows:

```
> require('reddit-api');
Error: Cannot find module '.\api\account'
    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 E:\code\node\node_modules\reddit-api\index.js:247:7
    at Object.<anonymous> (E:\code\node\node_modules\reddit-api\index.js:252:5)
    at Object.<anonymous> (E:\code\node\node_modules\reddit-api\index.js:254:4)
    at Module._compile (module.js:456:26)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
```

I believe this is due to the path seperator being used, as the attached pull request fixes the problem for me. This will still produce paths such as `./api\category` on windows but oddly enough this does not produce any errors.