Add IContainerContext interface

[ENikS]

Add 'official' inward looking interface to the container.

This interface should be a standard (same as IUnityContainer is for outside world) internal window to the container's engine.

/// <summary>
/// Container interface exposing engine to internal container parts 
/// </summary>
public interface IContainerContext
{
    /// <summary>
    /// The container that this context is associated with.
    /// </summary>
    /// <value>The <see cref="IUnityContainer"/> object.</value>
    IUnityContainer Container { get; }

    /// <summary>
    /// The <see cref="ILifetimeContainer"/> that this container uses.
    /// </summary>
    /// <value>The <see cref="ILifetimeContainer"/> is used to manage <see cref="IDisposable"/> objects that the container is managing.</value>
    ILifetimeContainer Lifetime { get; }

    /// <summary>
    /// The policies this container uses.
    /// </summary>
    /// <remarks>The <see cref="IPolicyList"/> the that container uses to build objects.</remarks>
    IPolicyList Policies { get; }

}

This interface is different from ExtensionContext in usage pattern. ExtensionContext exposes Strategies, Defaults and etc. and mainly used to configure and extend the container.
This inerface exposes only parts required for operating already configured container.