consistency between usermeta keys whether linking or registering

The usermeta keys for `oauth_token` and `access_key` are misnamed and used inconsistently in the twitter oauth class. Sometimes `_twitter_oauth_token` is a string (the `oauth_token` itself) and sometimes it's an array (the `access_key`, which contains the token and secret).

Confusingly, the code expects `_twitter_oauth_token` to always contain the `access_key`! If it's just a string - which you'd be forgiven for thinking it was - the connection will always fail when set\_user() attempts to connect to twitter using the following code:

````
$this->access_token = get_user_meta( $this->user->ID, '_twitter_oauth_token', true ); 
$this->client = new TwitterOAuth( $this->api_key ,  $this->consumer_secret, $this->access_token['oauth_token'], $this->access_token['oauth_token_secret']);
````

You can see the problem in `link()`, where it says
```
update_user_meta( get_current_user_id(), '_twitter_oauth_token', $this->access_token['oauth_token'] );
/* ... */
update_user_meta( get_current_user_id(), '_twitter_access_token', $this->access_token );

```

and in `update_user_twitter_information()`, called by `register()` and `login()`, where it says:
````
update_user_meta( $this->user->ID, '_twitter_oauth_token', $this->access_token );
update_user_meta( $this->user->ID, '_twitter_access_token', $this->access_token['oauth_token'] );
````

This P.R. changes the keys in `link()` so they match up with the others. apologies for misnamed commit (s/postmeta/usermeta/g)