Scan an assembly in order to create a DataContractContentSerializer

Resolves #100.
>or make these two tests more readable

I'd like to try this first, as it doesn't require changes to the SUT.

How about doing *something like* this:

```c#
[Theory, AutoData]
public void ScanCorrectlySerializesAttributedClassInstance(
    DataContractTestEventX @event)
{
    var sut = DataContractContentSerializer.Scan(@event.GetType().Assembly);
    var actual = @event.AsSerializedString(sut);
    var expected = XDocument.Parse(
        "<test-event-x xmlns:i=\"http://www.w3.org/2001/XMLSchema-instance\" xmlns=\"http://grean.rocks/dc\">" +
        "  <number>" + @event.Number + "</number>" +
        "  <text>" + @event.Text + "</text>" +
        "</test-event-x>");
    Assert.Equal(expected, XDocument.Parse(actual), new XNodeEqualityComparer());
}
```

The `AsSerializedString` is a [Feature Envy](http://c2.com/cgi/wiki?FeatureEnvySmell) defined as:

```c#
internal static class Conversions
{
    internal static string AsSerializedString(
        this IDataContractTestEvent @event,
        IContentSerializer serializer)
    {
        var sb = new StringBuilder();
        using (var w = XmlWriter.Create(sb))
        {
            serializer.Serialize(w, @event);
            w.Flush();
        }
        return sb.ToString();
    }
}
```