fix old reference to $element

data-uk-button triggered error when clicked due to old reference to $element
renamed keyframe animation uk-spin to uk-rotate
added position classes
fixed height viewport variable
Added new keyframes and classes to animation component
Cleanup
fixed scale keyframe animation
docs updated
docs updated
fixed uk-animation-bottom-center
responsive tabs updated
updated docs - animation
Merge branch 'master' of github.com:uikit/uikit
animation docs js updated
added flex add-on, updated utility, animation
Merge branch 'master' of github.com:uikit/uikit
Fixed animation reverse timing function
Merge branch 'master' of github.com:uikit/uikit
updated docs - added flex add-on to off-canvas navs
Merge branch 'master' of github.com:uikit/uikit
snippets regex: fixed matching empty lines. fixed class names place holder.