Xamarin Component Store Compatibility

I'm working on the Xamarin Component Store team and am trying to help get SignalR to a state where we can ship it as a component.

This pull request includes a bunch of changes (leaving the old work in place for now) that will bring SignalR and the samples into compliance for acceptance to the Xamarin Component Store.  This includes logos, an artwork banner, tweaking of the components.yaml file, a powershell build script, a readme for how to build the component, and also samples for iOS/Android/WP8. 

I've copied the samples over from the root samples folder and tweaked them for the component so we don't have issues with NuGet targets on Xamarin Studio, and so they're organized in a way that is compatible with the component.

We'd like to get this into the component store asap.  We're more than happy to submit the component on your team's behalf, I just need to know that you'll be happy with the submission as it is in this pull request!