Added location support to Twitter and Facebook

Hi,

I'd like to use Xamarin.Social for sharing pictures from a camera component in an app. I noticed that it yet lacks location support. I added my humble idea of location support and would like some input. 

* I was only able to test it on iOS (6 & 7) for lack of an actual Android test device and license.
* I would like to add it to App.net as well but am not yet ready to invest $100 for the developer account just yet.

They It currently works is, that the app determines it's location and assignes lat and long values to the Location property of the item. If the values are set, we pass them verbatim to the Twitter Api. For Facebook we have to find a place_id first. An additional request is necessary for this. I imagine the API for App.net being similar to Twitter in this regard.

michael