Warn on invalid properties when using `set_attributes` (:warn or higher)

* PM::Logger updated -- :verbose is more verbose than :debug (it's basically way over the top).
* If you use :debug log level or higher, `set_attributes` now shows debugging info when you're trying to apply properties that don't exist on the target element. This helps with debugging quite a bit. For example, if you misspelled a property, it used to just fail silently. Now it'll show you some information that there's something going on.
* If you set :verbose log level, it'll additionally show a backtrace. This is really slow, though.

![screen shot 2014-09-15 at 6 06 56 pm](https://cloud.githubusercontent.com/assets/1479215/4281260/dad5991e-3d3d-11e4-822a-edf9028277f7.png)


This is sllloooowwwwwww but very helpful sometimes.