GFFFile.py: collect file format and version in GFFFile, and make available via 'format' and 'version' properties.
GTFFile.py: set format to 'gtf' in GFTFile instances.
GTFFile.py: implement __iter__ built-in for GTFAttributes class (plus basic test cases).
GFF3_Annotation_Extractor.py: further attempts to generalise for GTF input.
Added example scripts and data in 'examples' subdirectory.
Bumped version to 0.8.2.