Support DateTimeOffset as X value

The primary motivation here was to support `DateTimeOffset` as the X value (and potentially other custom dates). This matters a lot in financial applications - because everybody has different ways of dealing with dates (and using `DateTimeOffset` or custom date is the most frequent).

The problem is that .NET charts only support `DateTime`. So, this pull request adds a simple step that converts keys (X values) using a "convertor" that can be registered by the user (similarly to `fsi.AddPrinter`).

In summary:

 * I made a distinction between X values (`#key`) and Y values (`#value`)
 * I added functions that automatically convert keys & function `Chart.RegisterConvertor`
 * I also deleted unused file `FSharp.Charting.AspNet.fs` (ASP.NET is done using `#if WEB`, right?)