add CSS will-change: transform to the transformation plane

# Problem
Scrolling performance takes a serious hit in Chrome 39+ due to the translateZ hack no longer working.

# Solution
Tell the browser we really want a GPU layer by adding CSS will-change: transform to the transformation plane

# Testing
- Confirm poor scrolling performance with a large list of items in Chrome 39+ (Canary 41 is easy to install side by side)
- Using this PR branch, confirm that the div#awesomeMap-transformationPlane has a style of `will-change: transform`
- Confirm scrolling performance is back to being awesome