added Material Design color palette for .panels in .less source files

Hi,
It's me again, an update of my previous PR to add MD color palette for .panel
I've edited the source less files like you asked. I didn't use the mixin you advise because there's more than one color for panels, and I didn't want to handle Bootstrap default color palette (primary, info, etc). I just used the same method used in original Bootstrap, but for Material Design color palette.
I hope it's ok now, tell if if there's anything wrong.
Instead of declaring the border color here, you may use `darken(spin(@border, -10), 5%);` inside your mixin (I think, if LESS allows this)