Implementation of list_insert_after
fixing up list_insert_after; handle NULL for node as pushing to front, fix comments in list.h, add tests
push front/back simply wrap list_insert_after
fix documentation
Merge pull request #15 from abellenir/list-insert-after

Full implementation of list_insert_after
Added function for getting the number of references to an object
fix memleak in list_delete_node
Merge pull request #17 from abellenir/memleak

closes issue #16