Fixed a cloning bug in ValueWalker.
Fixed a bug in reporting attributeValue events.

The bug would make it so that if multiple attributeValue events were
possible, only a single one was reported.
Dependency update.
Drop support for node 0.8.
Version bump.
Merge branch 'release/0.21.0'
Merge tag 'v0.21.0' into develop

v0.21.0