Readme update and decorate message for external callback

<a href='#crh-start'></a><a href='#crh-data-%7B%22processed%22%3A%20%5B%22https%3A//github.com/AzureAD/azure-activedirectory-library-for-objc/pull/238%23issuecomment-66856329%22%2C%20%22https%3A//github.com/AzureAD/azure-activedirectory-library-for-objc/pull/238%23issuecomment-66868524%22%2C%20%22https%3A//github.com/AzureAD/azure-activedirectory-library-for-objc/pull/238%23issuecomment-67082073%22%5D%2C%20%22comments%22%3A%20%7B%22General%20Comment%22%3A%20%7B%22html_url%22%3A%20%22https%3A//github.com/AzureAD/azure-activedirectory-library-for-objc/pull/238%23issuecomment-66856329%22%2C%20%22comments%22%3A%20%5B%7B%22body%22%3A%20%22Why%20is%20Travis%20failing%3F%22%2C%20%22created_at%22%3A%20%222014-12-13T00%3A35%3A35Z%22%2C%20%22user%22%3A%20%7B%22avatar_url%22%3A%20%22https%3A//avatars.githubusercontent.com/u/4307330%3Fv%3D3%22%2C%20%22html_url%22%3A%20%22https%3A//github.com/RandalliLama%22%7D%7D%2C%20%7B%22body%22%3A%20%22Fixed.%20%3A%2B1%3A%22%2C%20%22created_at%22%3A%20%222014-12-13T07%3A37%3A17Z%22%2C%20%22user%22%3A%20%7B%22avatar_url%22%3A%20%22https%3A//avatars.githubusercontent.com/u/5713915%3Fv%3D3%22%2C%20%22html_url%22%3A%20%22https%3A//github.com/kpanwar%22%7D%7D%2C%20%7B%22body%22%3A%20%22%3Ashipit%3A%22%2C%20%22created_at%22%3A%20%222014-12-15T22%3A53%3A45Z%22%2C%20%22user%22%3A%20%7B%22avatar_url%22%3A%20%22https%3A//avatars.githubusercontent.com/u/4307330%3Fv%3D3%22%2C%20%22html_url%22%3A%20%22https%3A//github.com/RandalliLama%22%7D%7D%5D%2C%20%22title%22%3A%20%22General%20Comment%22%7D%7D%2C%20%22approved%22%3A%20%7B%22https%3A//github.com/RandalliLama%22%3A%20%7B%22avatar_url%22%3A%20%22https%3A//avatars.githubusercontent.com/u/4307330%3Fv%3D3%22%7D%7D%7D'></a>
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- [x] <a href='#crh-comment-General Comment'></a> <img src='http://www.codereviewhub.com/site/github-completed.png' height=16 width=60>&nbsp;<b><a href='https://github.com/AzureAD/azure-activedirectory-library-for-objc/pull/238#issuecomment-66856329'>General Comment</a></b>
- <a href='https://github.com/RandalliLama'><img border=0 src='https://avatars.githubusercontent.com/u/4307330?v=3' height=16 width=16'></a> Why is Travis failing?
- <a href='https://github.com/kpanwar'><img border=0 src='https://avatars.githubusercontent.com/u/5713915?v=3' height=16 width=16'></a> Fixed. :+1:


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