let all nconf options can be overwrite by corresponding environment variables.

the idea is generating default values of nconf by using makeDefaultConfig function, and makeDefaultConfig sets proper values if environment is present or not.

makeDefaultConfig receives an object includes pairs of environment variables and default values.

## Impact

rename DB_PATH to DATABASE_PATH , so user can user DATABASE_PATH to overwrite {'database': {'path':''}}
rename twitter_url to twitter_callback  in nconf, so user can overwrite it by setting TWITTER_CALLBACK environment variable.