Add environment variables YAML config for Applications

fixes #569 
add optional "environment" key to application YAML config
a dictionary of environment variable mappings to be made
available to the application's container
I thought that pyyaml decoded the supplied file as utf8 and that the type should always be unicode:
 * http://pyyaml.org/wiki/PyYAMLDocumentation#LoadingYAML

But if that isn't the case, perhaps it's better to do `isinstance(value, typss.StringTypes)`
 * http://pyyaml.org/wiki/PyYAMLDocumentation#LoadingYAML

Also check with @itamarst or @exarkun and see what they recommend.