Retry block

* Added transient fault handling application block
* Implemented retry handler with default Exponential Backoff retry policy (3 times starting from 1 sec and backing off by 2 sec)
* Updated ServiceClient to automatically inject retry handler to HttpPipeline
* Added unit tests<p><a href="https://github.com/WindowsAzure/azure-sdk-for-net/pull/369">https://github.com/WindowsAzure/azure-sdk-for-net/pull/369</a></p>
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