Minor changes in documentation about Polygons.

More precisions about how a polygon is constructed: external angles are used,
which is not obvious.
Merge pull request #228 from alexDarcy/precisions

specify use of external angles in Polygons
Allow semigroups-0.16
CHANGES for 1.2.0.6
Bump JuicyPixels to allow 3.2
Add BackendBuild class.