Update Invoice.cs

Dear Mr. Rowley,

For a couple of weeks now, I've been using Xero c# API you wrote (at first XeroApi.Net and now Xero-net) on a project I'm working on. Since I needed BrandingThemeId and Url properties for the Invoice class, I added them to the source on my own. However, since the Xero-net API is a work in progress, and I'd like to have all the features there are, I was wondering, is there any way you could add those two properties to your source code?

(A day before you posted PDF support for invoices and credit notes, I made my own way to get a Pdf from Xero, because I needed the feature. The day after I realized you made a better and much more robust way of doing the same, so I updated the API based on the changes you made. So, I concluded it's simpler to ask you about these properties then to constantly pay attention on them on each new API update.)

Thank you for your consideration and the efforts to improve the API.
Sasha