add const char* gt_version(void) to API
put gt_fasta_show_entry() into FASTA module

fixes a documentation problem
GFF3 output with "retainids": multi-features

Make sure the GFF3 output of multi-features with "retainids" is always correct
and does not depend on the position of the representative. This fixes #115.
Merge pull request #117 from gordon/multifeature

GFF3 output with "retainids": multi-features
use tab
Merge pull request #116 from gordon/version

add gt_version() and fix documentation problem