Using a list instead of an iterator

Convert the iterator seqref.keys() into a list and put that into refkeys. Otherwise, "if not bedfield[0] in refkeys:" will always complain the chromosome is not in refkeys. This is probably due to recent changes in Biopython.
Merge pull request #2 from yfu/patch-1

Using a list instead of an iterator in seqref.keys(), proposed by yfu