fast loading of a new btree from a sorted list with classmethod bulkload()
fast loading of a new btree from a sorted list with classmethod bulkload()

still todo: check that the provided list is sorted. currently it will happily
create a busted (not sorted) btree object.

teepark/btree,pombredanne/btree-1,teepark/btree,pombredanne/btree-1