Azure DocumentDB sink

Notes:

* Not thoroughly tested yet
* Not batching yet although this should be possible to add using a stored proc as per http://azure.microsoft.com/en-us/documentation/articles/documentdb-get-started/
* The SDK is pre-release so likely to change over time - I wonder if we should lock the nuget dependency version for now to avoid later versions breaking this sink?
I copied this from the Raven stuff. Not sure if it's worth pulling it out somewhere?