moodle-tool_mergeusers - #20 : user_lastaccess table to be merged

Related to issue #20:

* user_lastaccess was excluded from merging till now. As Luis de Vasconcelos suggested,
  this table should be included to be merged in order to make database consistent.
Merge pull request #33 from jpahullo/issue20

moodle-tool_mergeusers - #20 : user_lastaccess table to be merged