update contributors
Use GPGME::Data.empty! when available
fix_encoding on person fields
dont fix encode if nil
Sort containers by message date and id
Merge pull request #184 from rakoo/discussion-order

Sort containers by message date and id
Fix deprecation of Enumeration::new
add history entry of thread sorting
Merge pull request #185 from rakoo/fix-deprecation

Fix deprecation of Enumeration::new
fix_encoding in text_mode
Prepare for 0.15.1
Merge branch 'develop'
Release 0.15.1