Added Handle-based ops to get/set/remove extended attributes in the libgfapi.

Change-Id: I1a8e666018d7b93e0bba2d9882935681da909980
BUG: 1089414
Signed-off-by: Soumya Koduri <skoduri@redhat.com>
Reviewed-on: http://review.gluster.org/7308
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Vijay Bellur <vbellur@redhat.com>