Added "includeRoot" plug to GafferScene::SubTree.

This means that when choosing /path/to/theNewRoot as the root, the subtree will be output under /theNewRoot rather than /. Previously we were performing an Isolate followed by a Subtree to achieve the same effect, so this is decent improvement in terms of convenience.

Fixes #565.
Merge pull request #694 from johnhaddon/subtreeRoot

Added "includeRoot" plug to GafferScene::SubTree.