DllImport SteamClient() and SteamClientGameServer()

These will be used to make sure that Steam is initialized.
Updated the header with some additional information
Don't error out in Packsize on a non supported platform

This allows Steamworks.NET to build for other platforms without modification.
Added version information in Steam.cs
Added an argument to Set() so that it behaves more like C++

This lets you set the function when you Set() the hAPICall.
This change is backwards compatible.
Now throwing exceptions if you use Steamworks when it's not initialized
Update PayPal Address
Updated CSteamworks to v1.4-1.28