Add object + content type constructor

[peschkaj]

The current constructor (string, string, object) assumes a Content Type of JSON. This leaves something to be desired. The next ctor assumes (string, string, string, string), which is also undesirable. A new constructor that looks like the following should be created

public RiakObject( string bucket, string key, object value, string contentType)

[peschkaj]

@OJ has suggested RiakObject.CreateAsProtoBuf() and RiakObject.CreateAsJson() to wrap regular ctors so that RiakObject users don't need to mess around with a pile of constructors.