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?
On seconHmm on second thoughts it looks like (using the .NET SDK at least) you have to pass an object. Thoughts on the best approach given that?