Fix rfc3986 error

Referencing #126, this error is being caused by an object being passed into `ShareUtils.prototype.is_encoded`.

This is because `default_description()` and `default_image()` were returning objects instead of the strings. These were being set on the `description` and `image` keys in the config object for each network resulting in something that looked like this:

```javascript
{
  networks: {
    twitter: {
       description: {
         description: 'Description copy'
       }
    }
  }
```

This breaks everything because the rest of the code is expecting a string here. Including `ShareUtil.is_encoded` which is passed `@config.networks.twitter.description` in `normalize_network_configuration`.

Also, the test suite wasn't working because of a breaking change when moving to Coffeescript 1.7+. I fixed that, but the tests are still not passing, but wanted to submit this patch before tackling that. I did verify things were working in `index.html`.