Expose FastaRecord __iter__ method for hybrid line/object-based iteration

[mdshw5]

Sometimes it would be useful to do something like:

for line in FastaRecord:
  do something with the line

Currently FastaRecord's __getitem__ returns the entire contents of a FASTA entry as a string. Maybe a __iter__ method could return smaller chunks to limit memory consumption. Might be able to use the read-ahead buffer from #26.