Updated mixins to use browser prefixes. Added animation mixin

Changed addprefixes() mixin to prefix() (line 4).
Updated box-shadow() mixin to include prefix (line 20).
Removed linear-gradient() and image-linear-gradient() mixins since the
gradient() mixin already has parameters that allow for those two former
mixins (line 24).
Added gradient-three-colors() mixin (line 61).
Updated transition() mixin to include prefixes (line 68).
Updated all transformation to use prefix() mixin (lines 72-92).
Updated background-size() mixin to use prefix() (line 111).
Added animate() mixin (line 259). Please read the comments on lines
250-258 to understand how to use the mixin correctly.