Adds 'decoratable?' method to Draper::Decoratable

Indicates whether a class can be decorated or not without resorting to a
rescue in application-level code.  I needed to deal with decoratable and non-decoratable models in the same area, so this change pushes the rescue block down and out of sight.  I wasn't able to get rid of the rescue entirely, as I'm sure you've found yourselves, due to the decorators not being defined until they're autoloaded.