Moved inverse functional identifiers into their own sub-section

This change started as an attempt to clarify that actors do NOT require inverse functional identifiers, rather they require a valid agent or group -- which in turn require inverse functional identifiers except for anonymous groups. In order to help clarify that I gave inverse functional identifiers their own sub-section, since talking about IFI's being required in the actor section makes it sound like they're required for all actors.
minor changes based on feedback

* fixed a typo
* re-worded definition of Inverse Functional Identifier
Merge pull request #147 from bscSCORM/actor

Moved inverse functional identifiers into their own sub-section