Add len method to gff object
Added transcript.total_exon_length method
added gene.longest_transcript_by_exon_length()
Added gene.remove_all_but_longest_transcript()
Deal with polypeptide records properly, by including them in transcripts
added script gff_keep_longest_transcripts
Merge pull request #4 from martinghunt/keep_longest_transcript_script

Keep longest transcript script