feat(anim): add animation events const
feat(anim): add animation events const

adds a constant providing all possible events to be raised by the concrete Animator implementations

part of issue https://github.com/aurelia/animator-css/issues/13

davismj/templating,JeroenVinke/templating,charlespockert/templating,AshleyGrant/templating,hitesh97/templating,milunka/aurelia-templating,aurelia/templating,michaelgeorgeattard/templating,jw56578/templating,PWKad/templating,simudream/templating,aurelia/templating,cmichaelgraham/templating,tcollard/templating,shanonvl/aurelia-templating