Add platform support for Windows Universal App (Windows 8.1 and Windows Phone 8.1) ; PCL friendly data layer

Xamarin Store Windows 8.1 and Windows Phone 8.1 Applications using the new Universal Apps project.
Also we take full advantage of C# and XAML in a great Visual Studio environment with design time support.

Existing Shared project is a new PCL project to be reused more easily and avoid linking files. For this task, i did some of the work for making it work, and later also used somethings like MD5 , PCL storage from the existing PCL branch on the project.

As the time the app does all the steps, but want finish the order (because isn't validated).
There are some other small things like animations, the kenburnseffect, and others like removing items from the cart that aren't yet implemented. Will work on that until the deadline.
 
Windows Phone 8.1 Screenshots

![home_wp](https://cloud.githubusercontent.com/assets/1235097/2992198/e28462d4-dc8b-11e3-9670-932dfaf6e00a.png)
![main](https://cloud.githubusercontent.com/assets/1235097/2992200/e2898e58-dc8b-11e3-8d4b-d7cb6afd2c10.png)

![checout_wp](https://cloud.githubusercontent.com/assets/1235097/2992202/e28e83c2-dc8b-11e3-896e-be8249078de0.png)
![detail_wp](https://cloud.githubusercontent.com/assets/1235097/2992201/e28e81ce-dc8b-11e3-903c-b8dd4a403ef2.png)
![login_wp](https://cloud.githubusercontent.com/assets/1235097/2992211/1cceab34-dc8c-11e3-8f64-65a5171f45e7.png)

Windows 8.1 Screenshots

![main_win](https://cloud.githubusercontent.com/assets/1235097/2992249/0bc287a6-dc8d-11e3-9de8-e871a4fba41b.png)
![detail_win](https://cloud.githubusercontent.com/assets/1235097/2992250/10cff468-dc8d-11e3-92a4-014b154e1b88.png)
![checout_win](https://cloud.githubusercontent.com/assets/1235097/2992255/17eb2d08-dc8d-11e3-9adf-f8974de4a73a.png)
![login_win](https://cloud.githubusercontent.com/assets/1235097/2992256/1db83cda-dc8d-11e3-924a-94376b020255.png)

Visual Studio /Blend Support at design time

![xamarinwin](https://cloud.githubusercontent.com/assets/1235097/2992199/e2897b84-dc8b-11e3-82b5-3e0890f7c469.png)