update
rework GFF3 export code
make mapped character list public
add test for GFF3Encoder
add tests for GFF3AttributeBuilder
remove obsolete comment
make aggregators tied to pre-mapping keys take precedence
Merge pull request #3 from satta/gff

Changes in GFF3 export
ignore attributes with null values
use reserved attribute list in GFF3AttributeBuilder.reserved_a
Merge pull request #183 from tcarver/master

Sascha GFF export and GFF encode / decode changes