Partial Classes for Column and MutableDataTable

Setting the class definitions for `Column` and `MutableDataTable` to be partial classes. 

Reason: allows a consumer of the nuget to define partial class files that can extend the definition of these classes. The specific use case that I am thinking of is that I want to serialize `MutableDataTable` objects using [protobuf-net](https://code.google.com/p/protobuf-net/) (while saving them in Redis). In order to do this, I need to set a `[ProtoContract]` attribute on the class to be serialized, and `[ProtoPartialMember]` attributes on all properties to be serialized (which can be done through partial classes).