[Bf-python] Mesh Layers

[Campbell Barton]

for people who dont read the CVS logs, heres some additions that may need some looking into.

Variable names an functionality 

exposed CustomData_get_active_layer_index in BKE_customdata.h (needed by python to get the active layer)
  
  added python api stuff to deal with Color and UV/Image layers.
  
  me.activeUvLayer - int
  me.activeColorLayer - int
  me.totUvLayers - int
  me.totColorLayers - int
  
  me.addUvLayer()
  me.addColorLayer()
  me.removeUvLayer()
  me.removeColorLayer()
  
  Variable names may need changing.


At the moment adding always makes a new layer at the end array and removing always removes the active layer.