write script to autoprefix BuildingBlocks

[gasolin]

The goal is generate cross platform friendly version instead of add additional cross_platform.css.

Since myth does not allow pass browser options.
use provecss could generate browser prefixes for
last 2 versions,Firefox ESR,Opera 12.1,BlackBerry 10,Android 4

We could evaluate with following command

npm install -g provecss
provecss headers.css headers.out.css --browsers all

It will add missing webkit- prefixes and keep existing moz- prefixes. So we could keep BuildingBlocks in gaia and generate cross platform friendly version.

What we need to do is

1. write a bash script or grunt command to automate this conversion.
2. remove cross_browser.css and use preprocessor