fix addNode to return true when node is added to anchor

[Levizzel]

Hey,
as mentioned in #113 the addNode function returns false every time when the node is added to an anchor.
In the changes from v0.6.3 true is only returned if the node is added to the scene and not if the node is added to an anchor.

I added the return for each Node-Type in Andorid and iOS.
I have checked it for Andoroid but can't do it for iOS.

[CariusLars]

thanks for the fix @Levizzel!

[CariusLars]

Could you write an entry for your fixes into changelog.md and set a new minor version number in the readme and the pubspec.yaml? Happy to merge after that :)

[Levizzel]

Done @CariusLars. I never contributed to a repo so I hope 0.7.0 is the right version number

[CariusLars]

@Levizzel could you actually make it 0.6.5 instead? Sorry for the back and forth, that one was my fault, 0.7.0 would be a new minor version (which is what I asked previously) but I now realized this should only be a patch, so it should be 0.6.5

[Levizzel]

@CariusLars Okay. Done!

[CariusLars]

just merged, thanks for the contribution @Levizzel !