hrcareer: create extension

We need to create an hrcareer extension based on the custom schema from here:

http://civihr.dev.civicrm.org/civicrm/admin/custom/group/field?reset=1&action=browse&gid=12

The ext will look a lot like hrident, hrmed, hrqual, or hrvisa, so there will be an XML file which defines the CustomGroup.

[FWIW, Yash is doing something similar for CiviVolunteer so you might want to coordinate with her.]

The wiki has some draft instructions here:

http://wiki.civicrm.org/confluence/display/CRMDOC43/Create+a+Module+Extension#CreateaModuleExtension-Addcustomfields%28CiviCRMv4.4+%29