delete resource, closes juan-medina#19

add resources, closes juan-medina#17

reuse entities, closes juan-medina#15

complete new listeners, closes juan-medina#13

implement goecs.Default, closes juan-medina#10

adding sort in place to view, closes juan-medina#8

reformat packages, closes juan-medina#5

fix bug juan-medina#4

add systems and listener as struct, closes juan-medina#3

add package level example