Add support for adding messages to the AppVeyor build log

[Philo]

Add the ability to send messages to the AppVeyor build log via the:

appveyor AddMessage command.

This could then be used to push specific messages (info, warning and error) to the separate AppVeyor messages tab for a build.

I would like to be able to combine this with the TaskSetup and TaskTeardown methods to log build script progress:

TaskSetup((context, task) =>
{
    var message = string.Format("Task: {0}", task.Task.Name);
    // custom logging
    if(BuildSystem.IsRunningOnAppVeyor) {
      BuildSystem.AppVeyor.AddInformationalMessage(message);
    }
});

TaskTeardown((context, task) =>
{
    var message = string.Format("Task: {0}", task.Task.Name);
    // custom logging
    if(BuildSystem.IsRunningOnAppVeyor) {
      BuildSystem.AppVeyor.AddInformationalMessage(message);
    }
});

Then further refine this into a common abstraction that can be agnostic of build system and can report messages specific to the current build environment (teamcity system messages, appveyor messages):

TaskSetup((context, task) =>
{
    var message = string.Format("Task: {0}", task.Task.Name);
    // custom logging
    BuildSystem.AddInformationalMessage(message);
});

TaskTeardown((context, task) =>
{
    var message = string.Format("Task: {0}", task.Task.Name);
    // custom logging
    BuildSystem.AddInformationalMessage(message);
});

[gep13]

@Philo Thank you for your contribution to Cake!