model/notification: Use builtin set() instead of deprecated sets module
model/notification: Use curly brace set syntax
config/environment: Added load_from_file() function

This commit merges much duplicate setup code for the environments of the
command line utilities.
Merge pull request #37 from Turbo87/deprecated-sets

Updated set usage to builtin set()
generate_assets.py: Use environment.load_from_file() function
Merge pull request #38 from Turbo87/config-defaults

config/environment: Added load_from_file() function
Merge pull request #39 from Turbo87/config-defaults

generate_assets.py: Use environment.load_from_file() function