MotionBase -> TimelineItem.

Doesn't change much (any) user-facing code.
Explains the point of the base class better.
Timeline owns unique ptr to all items on it.

Trying out for comparison.
Object creation/destruction seem to be faster (especially destruction).
Stepping the timeline is maybe faster; unclear.

I like the clarity of ownership.
Don't love the raw pointers in MotionOptions.
Unique ptr in MotionGroup.

Running into an issue with the Hola sample.
Pass unique_ptr by value.
Add virtual TimelineItem destructor.

Not sure how I missed that one before.
Merge pull request #4 from sansumbrella/uniqueness

Store unique_ptr's to TimelineItems.