Ability to transition inline elements

Inline elements automatically get switched to inline-block when
animated with the UI pack. This way transforms can take effect on them.
It is up to the user to ensure that they don’t transition span elements
if they don’t want them switched to inline-block. Closes #173.

Tweaked several effects to flow better.
:animating and transform value removal

Elements now get an “:animating” class when animating. Closes #172.

When transform properties hit their base values, they are removed
entirely from the element. Closes #155.