added basic support for attribute inerpolation. references #204
forgot them conditional comments
Merge branch 'attr'
attribute-animation docs. fixes #204
when un-forcing the height of the body, set it to an empty string.

Until now we were setting the height to `auto`, but this would override
the height specifid inside a stylesheet. Setting it to an empty string
will reset the style property.
Merge branch 'master' of github.com:Prinzhorn/skrollr
history.md
Merge branch 'master' of github.com:Prinzhorn/skrollr
history.md
0.6.24