fixed setOpacity in IE7 and IE8

setOpacity is not working in IE7 and IE8  

you can check it in the following link (changes the opacity each 2 seconds). It uses the leaflet 0.5 hosted in the CDN (I tested with the current master and i get the same behavior)

example: http://bl.ocks.org/d/4942923/
code: https://gist.github.com/javisantana/4942923

this is the version with the patch applied

example: http://bl.ocks.org/d/4942985/
code: https://gist.github.com/javisantana/4942985

I'm not pretty sure if that is the best way to manage the opacity for IE7 since the opacity for all the tiles should be changed to work.