Created NuGet package and added Apache license.

This is a nuget package for your bindings. I named the package Xamarin.Android.Picasso.JackSierkstra, so you would install it from the NuGet console via:
```
Install-Package Xamarin.Android.Picasso.JackSierkstra
```
You can push it to NuGet by building the project and then typing `nuget pack` and `nuget push` from the project directory. You will need a NuGet API key first.

Forgive me for adding the license - you can of course reject it. It was a field the NuSpec file wanted, so I added one.

In the above, I assumed you're not very familiar with nuget, forgive me if all of those things are obvious - they were unknown to me a couple of months ago.

Thank you very much for these bindings!