Implement ToString() for ZoneYearOffset, so that I can figure out why tests are now failing.
Implement ToString() for ZoneYearOffset, so that I can figure out why tests
are now failing.

BenJenkinson/nodatime,BenJenkinson/nodatime,jskeet/nodatime,zaccharles/nodatime,malcolmr/nodatime,malcolmr/nodatime,zaccharles/nodatime,zaccharles/nodatime,zaccharles/nodatime,nodatime/nodatime,zaccharles/nodatime,malcolmr/nodatime,zaccharles/nodatime,jskeet/nodatime,malcolmr/nodatime,nodatime/nodatime