allow export of GO Annotations within GFF3

[nathandunn]

The idea is to allow a user to write their own export class if they have a different version of GFF3, Chado, etc. and then have that available the Sequence Panel.

[nathandunn]

The initial context was to allow the ability to add a Chado exporter for exporting protein sequences.

[nathandunn]

FYI @satta

[nathandunn]

From @cmungall

General intro http://geneontology.org/article/gaf-gpad-and-lego

Specs: http://www.geneontology.org/specifications/gpad/gpad-1_2.html

[nathandunn]

From

On export the annotation ("GAF") file should include the user ID as well as the source.

[nathandunn]

related to #154