Correct Apache Amber to Apache Oltu

Apache Amber was renamed to Apache Oltu in December 2012 [1]. Correct
the names and links under OAuth 2.0 implementations.

[1]: https://mail-archives.apache.org/mod_mbox/incubator-amber-dev/201212.mbox/%3CCAAqLGLPwSEmcqPMsYk_wiCUtVbD9oZeQdFoAuL2HG5uf3f=Csw@mail.gmail.com%3E
Merge pull request #72 from charlessimpson/apache-amber-oltu

Correct Apache Amber to Apache Oltu