obama and raul , fidel castro 's brother , shook hands on friday .
white house says the two are due a ` substantive ' conversation today .
they met at the summit of the americas in panama alongside other leaders .
us ` is close to removing cuba from list of terrorist sponsor states '
move would automatically lift some economic sanctions on island nation .
the pair had shaken hands before at the funeral of nelson mandela in 2013 .
they will meet again today and discuss further thawing u.s.-cuba relations .
colombian president juan manuel santos hailed obama 's push to improve relations with cuba , saying it healed a ` blister ' that was hurting the region .