Updated Comments for intellisense

Howdy PayPal,
I've updated several of the comments in the SDK to help enhance the VS intellisense information.  If you could tell me which of the legacy APIs is the best to reference for the properties in the classes, I'd be happy to layer in more of the description of the properties/parameters, since I'm going to do it for my self anyways.  

I sent the pull request with only a few of the files updated thus far, because I'd like to have more of the more declarative descriptions from your other API docs implemented before I suggest the next pull. For a good example, please look at Api\Payments\CreditCard.cs The CVV2 property uses text from a previous API document.  Until I gain a better understanding of the interaction points between the classes, I left items that were not clearly identifiable (i.e. Type as string) as is.

Thank you for the SDK though, I slowed down a bit to document it for the other devs I work with, but so far I'm quite pleased with the implementation.  Keep up the great work.