Add debug mode for route constantization

[jcredding]

We should consider adding a debug mode that raises any NameError exceptions that occur while constantizing routes. This can be annoying to debug when you have a missing : or something else wrong that causes Deas to not be able to constantize the handler class name. We'd want this to be manually turned on via an environment variable or something else. For example something like:

DEBUG=1 rackup config.ru

could be useful.