moodle-tool_mergeusers - #37: moving button help from form to page header

Related to issue #37:

* A better place for the button help, considering John contributions,
I think it is better to place it in the header instead of the form.
Merge pull request #38 from jpahullo/issue37

moodle-tool_mergeusers - #37: moving button help from form to page header
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.