Added partner oauth and implemented the token renew feature

Please find below some notes on how to integrate and test the partner interface and renew token feature.

I have tested both items via Xero.Api.Example.Counts project.
1)	Please replace the clientcertificate.p12 (a file you would have got as part of your partner signup process).
2)	Public_privatekey.pfx (a self signed certificate generated during the partner signup process).
3)	Update your consumer key and secret and partner certificate password values in App.config.
4)	Run the project, and it will take you to the Xero Authorization page, after authorize step please get the verified code (Pin) and enter it on the command prompt (console window).
5)	Please feel free to put few break points to see how code works, the GetToken() method in TokenStoreAuthenticator will get the access token as well as handle the renew token process.

In future it would be great if there is a MVC web app example, which implements the “getAuthRedirectUrl()” method and integrates the oauth process and callback automatically.
In my app, I have implemented both partner oauth, callback and renew token process and it works great. 
Hope it helps others who are looking at integrating the partner oauth feature.
Cheers.