JSExportAttribute improvements

[jasongin]

Fixes: #269

This adds capabilities to JSExportAttribute to export all types from an assembly or exclude specific types or members from exporting:

[assembly: JSExport] // Export all public types in the current assembly (unless otherwise specified).

// This type is not exported: the type-level attribute overrides the assembly-level attribute.
[JSExport(false)]
public static class NonExportedClass
{
    // This method is exported as a static method on the module (not on the class).
    // The member-level attribute overrides any class or assembly-level attributes.
    [JSExport]
    public static void ModuleMethod();
}

// Without a type-level attribute, public types are exported due to the assembly-level attribute.
public class ExportedClass
{
    // The member-level attribute overrides any class or assembly-level attributes.
    [JSExport(false)]
    public void NonExportedMethod() {}

    // Without a member-level attribute, public members are exported along with the type.
    public void ExportedMethod() {}
}