Added `keys()` method to Settings and EngineSettings.
Refactoring - moved `user_agents.txt` to pipelines directory.
Don't initialize `RANDOM_USER_AGENT_LIST` from settings file.
Refactoring in `test_project` file.
Moved `save_response` signal to `signals` module.
`settings` command now dumps all the settings, when no option is specified.
Added dot at the end of the sentences in `short_desc()` of commands.