Add custom REST authentication module settings test & update doc comments in src

https://github.com/aerogear/aerogear-js/blob/master/src/authentication/adapters/rest.js#L93 is not covered according code coverage results.

I've added a test for custom REST authentication module to verify that the settings (baseURL, endpoints) are set correctly and updated the doc comments in source code since the below doc comments are wrong:

```js
auth.add( "module1", {
    baseURL: "http://customURL.com"
});
```
```js
auth.add( "module2", {
    endpoints: {
    enroll: "register",
    login: "go",
    logout: "leave"
    }
});
```

In addition I'd like to ask if it makes sense to check whether the baseURL ends with / and if not to add it automatically. Currently, if the baseURL is X and enroll endpoint is Y the result URL for enrolling is XY. Shouldn't it be X/Y?