Default host to localhost when in development mode.

* Running Rack apps on 0.0.0.0 in development mode will allow malicious
  users on the local network (ex: a Coffee Shop or a Conference) to abuse
  or potentially exploit the app. Safer to default host to localhost when in
  development mode.
Rack::Handler::WEBrick: default the host to localhost in development mode.
Rack::Handler::Mongrel: default the host to localhost in development mode.
Rack::Handler::Thin: default the host to localhost in development mode.
Determine the default value for Host, based on the environment.
Merge pull request #514 from postmodern/localhost_development

Default host to localhost when in development mode.