fix Position checking

fixes #274
unsigned long -> GtUword
Closes #278.
make hashmap methods optional to use

Closes #277.
remove unused hashmap declaration
remove unused functions
add -retainids to `gt merge'
accept and ignore empty input GFF3s in tidy modes
gtruby: fix FeatureNode.set_source
properly initialize string buffers

Closes #282.
warn when weird characters appear in argv

Closes #241.
Update README.md

add link to gitter chatroom
closes #230
make sure an error is set before printing it
update error message
cosmetics
add class(es) for sorted set of distinct numbers
use generated code instead of makros
add GT-Prefix to Makros