moodle-tool_mergeusers - #29 - supporting merging only if transactions are allowed.

Related to issue #29:

* Added setting to prevent from merging users if database does not support
 transactions.
* Added tricky way on MergeUserTool to get whether transactions are supported
 on curretn $DB object. See code for comments.
Merge pull request #40 from jpahullo/issue29

moodle-tool_mergeusers - #29 - supporting merging only if transactions are allowed.