Change semantics of keepWhen/dropWhen.

They no longer propagate the right Signal when the left Signal changes. Now they propagate events from the right Signal but only when the left Signal is True or False respectively.

For this example the counter only goes up when you click while hovering whereas now it goes up when you hover or click while hovering:
http://share-elm.com/sprout/52b7611ae4b06194fd2d364f