Create IHostingEnvironment and implementation

[glennc]

using System;

namespace Microsoft.AspNet.Hosting
{
    [AssemblyNeutral]
    public interface IHostingEnvironment 
    {
        string EnvironmentName { get; }
        string WebRoot { get; }
    }
}

This interface is the moral equivalent of the IApplicationEnvironment but for web applications. The environment name is the name of the currently running environment and the WebRoot is where the web sever is rooted.

[glennc]

@Tratcher Can you talk to @loudej about this before implementing, before Friday.

[Tratcher]

This includes #64, no?

[Tratcher]

See aspnet/dnx#613

[Tratcher]

Add a reference to Json.Net. Use the IApplicationEnvironment to find the project.json file and read it. Find the webroot element. Add IHostingEnvironment to the services.

For PhysicalFileSystem, retrieve IHostingEnvironment from the services and get the webroot. Fall back to IApplicationEnvironment.ApplicationBasePath like we do today. Consider moving this logic to the caller of PhysicalFileSystem (e.g. StaticFiles)