Added warning on erroneous styling methods (if log level is high enough)

I'd like to see a warning in the REPL if I'm trying to set an attribute that the object doesn't respond to.

![warn](http://media.clearsightstudio.com/jamon/1._bundled_rake_ruby_20140116_210948.jpg)

However, this will expose some inefficiencies in ProMotion, so we should be prepared to deal with those. For example, PM::TableScreen carelessly applies a lot of attributes to cells that it shouldn't:

![overwarn](http://media.clearsightstudio.com/jamon/1._bundled_rake_ruby_20140116_211112.jpg)

Feedback welcome.