Management Api Improvements

Hi Mike,

I've fixed a bug in Connection.cs to accommodate large > 32bit statistics from Rabbit and added a unit test demonstrating the fix. 

I've also changed the implementation of ClientProperties.cs and Capabilities.cs to allow for a variable list of client properties and capabilities; the actual list of properties is a function of the remote client connected to Rabbit and (for me at least) it's very useful to be able to see these extra properties. The original typed get property accesors have been left in place to maintain backward compatibility with existing code which uses this class. I've done this with a bespoke JsonConverter - I'm not sure this is the right way to achieve this.

I probably should have submitted these changes as two different pull requests.

Regards,

Daniel