archbishop of canterbury justin welby hail victims of kenya university massacre as martyrs .
thousands of catholics brave bad weather to hear pope francis lead easter sunday mass in the vatican .
pope gave sombre message in st peter 's square , praying for an end to the persecution of christians .
pope francis also commemorated students massacred by islamist militants at a university in kenya .
the pontiff made a nod to lausanne agreement between iran and the international community a nuclear accord .