moodle-tool_mergeusers - several improvements:

    * lib/autoload.php to enable autoloading of classes from the plugin. Removed lib/lib.php
    * moved locallib.php to lib/mergeusertool.php to enable autoloading.
    * updated README.txt for new Gatherings.
    * Ctrl+C is moved from Gathering to Merger class, to remove this responsibility from new Gathering implementations.
    * Detection of an incompatibility on special case on user_enrolments table: if a user A is merged into user B, but afterwards, user B has to be merged back into A, a conflict appears when processing the status == 2. It is weird, but it happaned to us. It now checks the status flag to know whether to update record to status = 0 (to reactivate a deactivated record), and status = 2 (to deactivate an activated record). See the lib/mergeusertool.php.