Pull request #667 breaks popups in IE

[ajbeaven]

A bit of contentious code here it seems.

Pull request #667 seems to have broken popups in IE. This happens because in the onAdd function in L.Popup, uses setOpacity we make opacity 0, and then later on sets it to 1. setOpacity as it is written now means that we can never set opacity to 1.

Based on the description, I think it may also break #600

Obvious suggestion would be to revert #667, but there may be a way to improve the code to resolve both issues.

EDIT:
Created pull request