Allow max_request_size to be tuneable.

Added envy as dependency to simplify application environment interrogation.
Extracted size enforcement to another module and added tests.  Application
environment is polled on every request allowing the configuration to be tuned
at runtime via application:set_env(chef_wm, max_request_size).
Merge pull request #78 from opscode/of/OC-9686

Of/oc 9686