Configuration files written in YAML

Maybe is YAML an option for the configuration files?

I also fixed a couple of bugs. The DoormanError exception raised an exception, and I implemented a configuration checker, which checks for (basic) syntax and if all files mentioned in the config exist. This checker needs some further expanding.

### Example configuration

    # social accounts
    /home/user/twitter.rb:
     twitter_passowrd: bHc0yz
     private_key: 1VpzKbLDTqC1vXb
    /home/user/pythoncodes/githubapi.py
     github_password: wKJ4cV

    # secret text
    /etc/secret.conf:
     my_secret: really secret thing

    # secret function
    ../settings/my_settings.py:
     my_func: [i for i in others if i < 3]