GH502: Refactored ICakeEnvironment

[patriksvensson]

This included obsoleting a lot of members and also
introducing two concepts, ICakePlatform and ICakeRuntime.

ICakePlatform is an abstraction around the actual platform
that Cake is running on. This will give you information about
things such the platform family and architecture.

public interface ICakePlatform
{
    PlatformFamily Family { get; }
    bool Is64Bit { get; }
}

public enum PlatformFamily
{
    Unknown = 0,
    Windows = 1,
    Linux = 2,
    OSX = 3
}

ICakeRuntime is an abstraction around the runtime that Cake
is running in such as the .NET target platform.

public interface ICakeRuntime
{
    FrameworkName TargetFramework { get; }
}

Closes #502

[dnfclas]

Hi @patriksvensson, I'm your friendly neighborhood .NET Foundation Pull Request Bot (You can call me DNFBOT). Thanks for your contribution!
You've already signed the contribution license agreement. Thanks!

The agreement was validated by .NET Foundation and real humans are currently evaluating your PR.

TTYL, DNFBOT;

[patriksvensson]

Wrong name of method.

[patriksvensson]

Should we add an extension method to ICakeEnvironment called IsUnix for convenience and backwards compatability? Not sure how we going to solve it since the current method on ICakeEnvironment needs to be obsoleted (?).

[patriksvensson]

@gep13 @devlead This one is ready for review.

[devlead]

@patriksvensson LGTM tested on several projects. I've rebased and merged into develop.