Small mesh/armature/animation improvements

[scurest]

• Includes my fix for #29. I don't know for sure if it is fixed by that, but we need the change I made anyway.
• Animation interpolation is set. Rotation always goes the short way round (try AnimatedTriangle for example).
• Restrictions about overlapping armatures/skeleton properties being required were lifted.
• And most importantly, the user can select a bone rotation! Try CesiumMan with XYZ rotations of 90d/0/0 and the model will actually look fairly good in Blender for once :)

@ksons Something bad happened when you merged the last PR and you lost the git history and people like cinghiale didn't manage to get credited as contributors. It made it really difficult to merge master into this branch and it's also why Github is listing all the commits from the last PR again. This PR has all the history intact so if you can do a normal merge this time that should be fixed :+1:

[ksons]

Reverted as I squashed the history which makes it difficult to merge the subsequent PRs.