Remove token and user id keys from session and local storage on logout

Remove 'accessTokenId' and 'currentUserId' keys from session and local storage on logout.
Fixes #61 
I'm not sure how to trigger page reload here and need your advice. To replicate the error on my end I do the following:
1. Login with parameter ```rememberMe = true```
2. Reload the page
3. Log out
4. ERROR: The keys are not cleared in the localStorage.

Here is what I think is happening: 

The default value of the ```LoopBackAuth.rememberMe = undefined```. It gets set in the callback of the ```login``` function and subsequently the ```$LoopBack$accessTokenId``` and the ```$LoopBack$currentUserId``` keys are stored in the localStorage:
```
LoopBackAuth.rememberMe = response.config.params.rememberMe !== false;
LoopBackAuth.save();
...
LoopBackAuth.prototype.save = function() {
    ....
    var storage = this.rememberMe ? localStorage : sessionStorage;
...
```
However, if users reload the page or close it and open again the value is set back to ```undefined```. Then on log out the ``` LoopBackAuth.save();``` function is called again and the ```storage``` variable now starts pointing to the sessionStorage. Hence, the keys are cleared in the sessionStorage, but left intact in the localStorage.

Please note that I'm using ```getCurrent``` function to get a reference to the currently active user after I reload the page or reopen the browser. I don't see the ```rememberMe``` property being passed in the response there. If it has been it would make sense to set ```LoopBackAuth.rememberMe``` there to fix the issue as well.