Support linear gradients using CSS3PIE

[walling]

The code background linear-gradient(top, 0% #46BFD3, 4% #3398B7, 100% #287790) will result in an extra property being generated -pie-background to support http://css3pie.com. This fixes #51.

[superstructor]

Looks good :-) would be nice to have some PIE in nib.

I think it is important to add the following before this can be merged

• make it configurable, code as-is will always include the -pie-background property
• add a pie() mixin to include the behavior, and maybe some pie position mixins too, much like Compass has already https://github.com/chriseppstein/compass/blob/stable/frameworks/compass/stylesheets/compass/css3/_pie.scss

PIE doesn't always work, especially for elements that are generated/animated/moving in many cases, even with the pie-watch and pie-poll hacks sometimes you just end up with element background gradients being in a different position to the element etc. Its also very slow on large elements e.g. full-page background. For these cases its important that its configurable so the developer can use another IE solution such as DXImageTransform gradients.

On a side note (out of scope of this pull request) it would be nice to add configurable DXImageTransform gradients to the linear gradient mixin, I might take a look at that...

[superstructor]

PIE support will need to be re-implemented on top of #94 when it is merged. This pull request should probably be closed ?

[vendethiel]

@slang800 close this one?