Enable to custom triangle shape

Use $width and $height to custom triangle shape, if omit $height, this
mixin will generate code in the old way. The only downside is that
$width value should always be even number, otherwise triangle width
will be short 1px.
Enable to custom triangle shape for up-left, up-right, down-left, down-right
Accept 2-element list for `$size` and `$color`

The first value in $size would be the width value, and the second would
be the height value. The first value in `$color` would be foreground
color, and the second woud be the background color.
Added "inset-up, inset-down, inset-left, inset-right" four arguments to $direction

Please see http://codepen.io/P233/pen/AtzIw for more details.
Update position add on to utilize false/null rather than unitless zero values.
Merge pull request #343 from dcalhoun/support-unitless-position

Update position add on to utilize false/null rather than unitless values.
Remove transition from flat button
Added -ms-interpolation-mode to image-rendering mixin
Merge pull request #291 from P233/master

More control over triangle shape/mixin.
Merge github.com:thoughtbot/bourbon