typo
Clean up mixins, fix #9100 (no ms-keyframes)

- Spacing tweaks in the general mixins
- Add comments to transform mixins about IE support
- Remove `-ms-keyframes` because only IE10+ supports it, and without
prefix
update modals demo code
more docs changes
Merge branch '3.0.0-wip' of github.com:twbs/bootstrap into 3.0.0-wip
Fixes #9073: add note to homepage about download differences
Fixes #9049: switch order of gradient mixin params to avoid problems for upgraders
Fixes #8786 (part 1): darken hover states on buttons
Fixes #8786 (part 2): new default button styles

Also relevant: http://www.youtube.com/watch?v=Ldyx3KHOFXw
Fixes #8770: mobile-first (stacked by default) horizontal dls