moodle-tool_mergeusers - #19 : process correctly quiz attemps

First step is to allow this plugin to extend easily
the functionality of merging user data. To do so,
we have developed the concept of TableMerger, which
decides how to merge records from both user in a per
database table basis.

* Created TableMerger interface and a generic implementation
  (GenericTableMerger) and the one for the user_enrolments table
  (UserEnrolmentsMerger).
* Added new default configuration setting 'tablemergers' where
  administrator describes for each database table the TableMerger
  that addresses that table. By 'default' we set the GenericTableMerger
  which processes any table that has not a specific TableMerger.
* Updated autoload.php to load correctly new classes.
moodle-tool_mergeusers - #19 : process correctly quiz attempts (II)

  With this commit we add a specific TableMerger for quiz attempts.
  This new TableMerger adds new settings to the plugin, with which
  administrators will be able to choose what to do with quiz attempts
  when merging users:

  a. Do nothing. Quiz attempts are kept to each corresponding user.
  b. Keep attempts from old user and remove those from the new one.
  c. Keep attempts from new user and remove those from the old one.
  d. Keep all attempts and renumber them by timestart column.
moodle-tool_mergeusers - #19 : updated README.md with new functionality.
moodle-tool_mergeusers - #19 : bug solved on renderer.
Merge pull request #53 from jpahullo/issue19

Issue #19 - quiz attempts addressed for merging