v5.1.6 (23 May 2013)

Changelog:

• BugFix: CleanPolygon function was buggy.
• Changed: The behaviour of the 'miter' JoinType has been
  changed so that when squaring occurs, it's no longer
  extended up to the miter limit but is squared off at
  exactly 'delta' units. (This improves the look of mitering
  with larger limits at acute angles.)
• Added: New OffsetPolyLines function
• Update: Minor code refactoring and optimisations

Also includes the following:
v5.1.5 (5 May 2013)

• Added: ForceSimple property to Clipper class
• Update: Improved documentation