@InjectExtra annotation

Added an @InjectExtra annotation (heavily inspired by your @InjectView one), with ability to declare it optional.

This is my first attempt with annotation and code generation, so if anything is suboptimal or just plain wrong, please tell me :)

Here is a use example:

    public class HelloActivity extends Activity {
        public static final String EXTRA_MANDATORY = "EXTRA_MANDATORY";
        public static final String EXTRA_OPTIONAL = "EXTRA_OPTIONAL";

        @InjectExtra(EXTRA_MANDATORY)
        String mandatoryExtra;
        @InjectExtra(value = EXTRA_OPTIONAL, optional = true)
        String optionalExtra = "Hello!";  // default value

        @Override
        protected void onCreate(Bundle savedInstanceState) {
            super.onCreate(savedInstanceState);
            setContentView(R.layout.hello_activity);

            Extras.inject(this);
            // Do something with extra values
        }
    }