Maintain original ordering of records

[jdidion]

For some applications, it would be nice to maintain the original order of records found in the fasta file. There is no minimum version specified in setup.py - if it were made to be 2.7, then this could be accomplished by using collections.OrderedDict in place of dict for the index. Otherwise a slightly more complicated implementation would be required. I'd be happy to implement either and open a pull request if you tell me what the minimum version is that you're trying to support.