#112 Add a yFlip default setting

This may not be applied correctly to all the things yet, but the
intent is to provide an easy way to set all of Flower's components
to work with a 0-at-bottom orientation, with a single variable change.
#112 Add scissorRect support for Group

This allows a group to support setScissorRect() and automatically
apply the rect to child props. It also, if your MOAI tree has
getScissorRect(), avoids returning clipped props in
getTouchableProp. Without that change, a group with a scissor rect
can report clicks on members that are invisible, and if the group is
in front of another prop, it can prevent that prop from receiving
clicks.
Give touch events references to the props that they report being on.

If you touchDown on a prop, then move, then touchUp on another prop, the
first prop still receives the touchUp event, and e.prop is the prop
receiving the event, even though that isn't the prop that the touchUp
occurred on. Add "e.active_prop" and "e.other_prop" fields to allow
UI elements to distinguish these cases.
#112 Allow tweaking default blend mode

The Moai default blend mode can be surprising if you're not
used to it, and allowing people to change the default blend
mode makes life easier.