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.
I used specific colors to be closed to what's done with original Bootstrap panels (see picture), while using Material Design color palette. A soft-tone background, a border slightly darker, with a stronger text color.
![panel_bs](https://cloud.githubusercontent.com/assets/9024503/4810349/aebda6fa-5eb2-11e4-9049-e1845e1e5c4b.png)
I wasn't aware of this specifications, I just checked the colors and tints. If I use @darkbg-text, every panel style will have the same text color. Is that what you suggest ?
Sorry, I'm a beginner.
Actually, at first I was using you project and just wanted to have a panel styled in -material-bluegrey, but it wasn't available. So I thought I'd add it, and eventually I decided to do it for the other Material colors and share it, in case somebody wanted to styled panels like me.
Does it fit with your project?