Version 0.8.0: move GTF handling classes into new module 'GTFFile.py' from GTF_extract.py.
GFFFile.py: minor update to how GFFAttribute prints out the 'no key' items.
GTFFile.py: update how attributes are handled in GTFDataLine, so the same syntax can be used interchangeably with GFFDataLines.
GFF3_Annotation_Extractor.py: updated to use GTFFile classes if input is GTF format.
Bump version to 0.8.1.