Added a targetOffset plug to the GafferScene::Constraint node.

This is an additional translation (in target space) applied to the target transform. It is of most use for the AimConstraint node, but since it has potential uses in the other constraints, and the Constraint base class specifies everything else about the target, it seemed most reasonable to implement it in the base class.

Fixes #278.
Merge pull request #709 from johnhaddon/aimConstraintOffset

Added a targetOffset plug to the GafferScene::Constraint node.