Skip list modifications and test.

Added functions
  skip_delete_node for properly deleting skip_node
  skip_destroy_list for properly destroying whole skip list
  skip_print_list for printing the skip list.

Changed return values of function skip_insert().
Renamed skip_delete() to skip_remove() and changed return value
  to int (successful / not found)
Bugfix in skip_remove().

Added test of skip list.