Add async support to Polly

I added asynchronous support to the .NET 4.5 version Polly. Doing external IO is one of the kinds of job where a retry policy is most likely to be effective at resolving an exception. As with .NET 4.5 that is typically done with async/await instead of with a blocking call.

Example usage:

    try
    {
        Result.Text = "Executing";

        var policy = Policy.Handle<HttpRequestException>()
                .WaitAndRetryAsync(new[]
                {
                    TimeSpan.FromSeconds(1),
                    TimeSpan.FromSeconds(2),
                    TimeSpan.FromSeconds(3)
                });

        var html = await policy.ExecuteAsync(() =>
                {
                    var client = new HttpClient();
                    return client.GetStringAsync("http://google.com");
                });

        Result.Text = html;
    }
    catch (Exception ex)
    {
        Result.Text = ex.ToString();
    }