Some fixes in EAD resourceRelation, fixes #4381

Squashed commit of the following:

commit 08c966d1b90588f0af409021ddcc8daa29e1ade3
Author: José Raddaoui Marín <raddaouimarin@gmail.com>
Date:   Mon Feb 18 06:20:32 2013 +0100

    Add resourceRelation subjectOf import

commit b58c89488e6224a91b5c86eae6d8e888e65183d7
Author: José Raddaoui Marín <raddaouimarin@gmail.com>
Date:   Mon Feb 18 03:49:37 2013 +0100

    Add resourceRelation subjectOf export

commit fc07aac401f71cf1bc9e5362aedac1aa8664af86
Author: José Raddaoui Marín <raddaouimarin@gmail.com>
Date:   Mon Feb 18 03:44:06 2013 +0100

commit fc07aac401f71cf1bc9e5362aedac1aa8664af86
Author: José Raddaoui Marín <raddaouimarin@gmail.com>
Date:   Mon Feb 18 03:44:06 2013 +0100

    Add xlink:role support in resourceRelation xml import

commit d5ef7648a3b81175bf754227accffa6cce9e9f37
Author: José Raddaoui Marín <raddaouimarin@gmail.com>
Date:   Mon Feb 18 03:34:38 2013 +0100

    Fix wrong attribute in resourceRelation export

commit 9cb5982155477e8013f8b5f4e6e8c985525ab8eb
Author: José Raddaoui Marín <raddaouimarin@gmail.com>
Date:   Mon Feb 18 03:32:58 2013 +0100

    Add xlink:role attribute to resourceRelation export