bug fix for LabelBMFont::getBoundingBox()

LabelBMFont::getBoundingBox() is the bounding box of LabelBMFont relative to its parent.

while _label->getBoundingBox() is the bounding box of _label relative to LabelBMFont.

Also the size of the above 2 boxes are the same, their offset are not necessarily the same.

(_label is always positioned at Point::ZERO of LabelBMFont, which is not necessarily the case for LabelBMFont)

This bug can cause that we can't correctly detect whether a LabelBMFont is clicked or not:http://www.cocoachina.com/bbs/read.php?tid=209406