Add config classes to provide keybindings The new Config class to be able to configure getan. Until now it is only possible to configure the Keybinding. The config is read from $HOME/.getan/getanrc and /etc/getanrc of the first file isn't found. If both files aren't available a default is used.

- Create empty databases from within getan.

- Create a 'Help-Widget' to give the user an information about all possible
  keys.

- reimplement RPN input for current getan implementation (28489e672e61)