Evict OAuth me when setting credentials.

This fixes a bug with multiple `set_access_credentials` calls
within `cache_timeout` with the 'identity' scope and
`update_user` not set to `False`. Because config['me'] is
cached, the subsequent calls will return the cached and
inaccurate result.

Bug found because of [this r/redditdev thread](http://www.reddit.com/r/redditdev/comments/1905ed/how_to_reuse_oauth_tokens_with_praw/)