Allow to preserve axis information when combining charts

This fixes #3
The way it was original fixed in FSharpChart was a little hackish, as it assumed the first chart was the one with the right values, which could not be the case.
So instead, I added a static method CombineWith which is more explicit on what values will prevail.

It allows for code like this:

    let baseChart =
        Chart.Point(data, Name = "Training Data")
        Chart.WithMarkers(Color.Red, 10, Style = MarkerStyle.Cross)
        |> Chart.WithXAxis(Title = "Population of City in 10,000s", Min = 4.0)
        |> Chart.WithYAxis(Title = "Profit in $10,000s")

    (...)

    Chart.Line(model, Name = "Model 1")
    |> Chart.CombineWith baseChart

    (...)

    Chart.Line(model2, Name = "Model 2")
    |> Chart.CombineWith baseChart