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?
If it is possible to send raw JSON to DocumentDB it'd be nicer to do that (using `JsonFormatter`) - not sure about the API support for this.

Using the formatter, rather than going through the simplification/serialization process, is a lot more efficient and produces much less garbage. Wondering how I might convert the Raven sink and others to use it as well....