Add support for getting the user's home directory in the Cake Environment

[augustoproiete]

The Cake Environment currently supports getting the WorkingDirectory as well as the ApplicationRoot, but doesn't support getting the user's home directory.

Proposed API:

public interface ICakeEnvironment
{
    /// <summary>
    /// Gets the user's home directory.
    /// </summary>
    /// <value>The user's home directory.</value>
    DirectoryPath UserHomeDirectory { get; }

    // ...
}