Adds a location Code to the location class of iTop

In our Information System, location are recognized by their names, but also by a locationCode (think, for exemple, at the Airport Code, at NYC, speaking of John F. Kennedy Airport or JFK Airport).

This very light contribution just add that : a Code field in the Location form, and a locationcode in all physical ou virtual equipments.

This is the catch : In our organization, Virtual Machines does have a location field, and this is provided by another extension (also available here) : iTop-VM-attrs (but this extension adds many more fields than just the 'location' to VirtualMachines, be warned :)

While hosted on this github page for the sake of simplicity, this is actually not my work but the work of one brillant coworker :