Update xpdogenerator.class.php
Bump version for 2.3.1-dev
Respect engine specified in mysql schema

Fixes modxcms/xpdo#42

Merge remote-tracking branch 'origin/pr/45' into develop

* origin/pr/45:
  Update xpdogenerator.class.php
Merge remote-tracking branch 'xpdo/master'

* xpdo/master:
  Bump version for 2.3.1-dev
  Update xpdogenerator.class.php