strip vendor prefixes from at-rules

[charabaruk]

• adds a function stripVendorPrefix(name) that checks for and removes vendor prefixes
• uses said function to remove vendor prefixes from at-rules to ensure fillType is correctly set

[saabi]

I'm delaying pushing this request for the moment as it affects the code copied from Tab Atkins' parser.

I've made modifications of my own but I'm discussing the possibility of moving these to his repository so I have yet to decide how to proceed with the project in relation to this.

[charabaruk]

Well, I've merged it into the current head of master in my own repo anyway, gimme a shout when you've decided what you'll do with this PR.