Visible alpha

(I'm new to contributing to github, so hopefully I'm doing this right.)

Here's another that allows configuring the alpha level when the popover is visible.

I actually needed this because there's a noticeable "stutter" in my app when initializing the popover.  So I'm using this with an alpha of 0.0f to cache the internal images/views/etc.

Oh, it looks like another thing got snuck in here... If you "#define WYPOPOVER_NOLOG 1" in your pch file, you can disable only the WYPOPOVER logs.  This should not affect the previous way logging behaved if not defined, however.