Implemented gbn_copy
Gave meaningful names to variables in sendto_.h
Moved code in sendto_.h to sendto_.c
Fixed various includes
Improvements to ringbufferwindow

* Added win_last_frame for convenience
* rbw_set_win_size now clears packets out-of-window if window shrinks.
* rbw_inc_head and rbw_int_to_head clear packets out of frame
* added rbw_get_n function for getting the offset of the given packet to head
gbnp_copy now copies all data
First implementation of GBNserver