v0.28

 - Add Blob -> id() (GH#15) (thanks, @hoelzro!)
 - Add TreeBuilder API (GH#16) (thanks, @hoelzro!)
 - Add Reference -> create() (GH#17) (thanks, @hoelzro!)
 - Add Repository -> refs() method (GH#19) (thanks, @hoelzro!)
 - Make Commit -> create() take an optional parameter indicating the name
   of a reference to update (GH#19) (thanks, @hoelzro!)