add a knownBug for iOS7 video autoplay, since its fixed in iOS8
feature detects for CSS ch, em and ex units
Merge pull request #1331 from jokeyrhyme/css-font-units

feature detects for CSS ch, em and ex units
Merge pull request #1370 from patrickkettner/autoplay-note

add a knownBug for iOS7 video autoplay, since its fixed in iOS8. Fixes #1038
use exec rather than spawn when using windows

fixes #1290

this gives us the least terrible of both worlds, using spawn unless
windows, where we use exec. That way everyone can build