* Added dependency on logrus.
* Command line flags are parsed in order to obtain the name of the
configuration file, the log level and the instance identifier.
* Logging in various places : configuration loader, API, data access.
Privilege computations and subsequent actions do not write proper logs
yet.
* The `username_attribute` configuration value was added to the `ldap`
section. When this value is set, the program will not try to extract the
username from DNs; instead, it will look them up and extract the
username from the referenced record, using the specified attribute.
* The program will no longer exit in error when a group listed in the
mapping doesn't exist.
