Make the ArangoDB connection configurable

We want to configure the following aspects of Guacamole through a convenient interface:

 * Database: The connection, with possible authentication, to ArangoDB. The following aspects should be considered: hostname and port, password, username and database name.
 * Logger: What logger to use. The logger should be used for the underlying `Ashikawa::Core` connection too
 * DefaultMapper: The default mapper class to use for mapping documents to models and vice versa