Consider supporting nullable task parameters

With a simple task:

```C#
public class DoNothing : Microsoft.Build.Utilities.Task {

public virtual bool? Parameter1 {
get ;
set ;
}

public override bool Execute() {
Log.LogError(Parameter1.ToString());

return Success;
}
}
```

and a simple project:

```xml

```

MSBuild errors, because it doesn't know how to handle `Nullable`, even when `T` is a nice, known type:

```
C:\Users\raines\foo.proj(22,16): error MSB4030: "true" is an invalid value for the "Parameter1" parameter of the "DoNothing" task. The "Parameter1" parameter is of type "System.Nullable`1[[System.Boolean, mscorlib, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089]]".
```