Declaring component attributes via annotations

As of #594, custom components can be declared to be useable in Facelets via the @FacesComponent annotation. Via this it's no longer required to have an entry in a taglib.xml file.

However, if the component author wishes to declare the component's attributes (for documentation, tooling, etc), XML still has to be used.

I therefor would like to propose declaring these attributes via annotations as well.

E.g.

```
@FacesComponent(value="components.CustomComponent", createTag=true)
public class Foo extends UIComponentBase {

@Attribute
String color; // will be injected with getAttributes("color")
@Attribute(description="This will determine the ...", required=true)
String style;

@Attribute(description="The cost of ... ", default="100")
Integer cost;

}
```