Specify that callers must had a reference to System.Xml; this is required by us implementing XML serialization.
Specify that callers must had a reference to System.Xml; this is
required by us implementing XML serialization.

Fixes issue 339.

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