Update how the chartLayout component calculates the width and height of DOM elements' content areas for improved cross-browser compatibility

This should fix the issue with Firefox, remain compatible with IE9+ and simplify the solution in #21.

There are some screenshots available on [BrowserStack](http://www.browserstack.com/screenshots/ed5123f23d9c940bc448b14640e5f7ed7abc3772) to demonstrate the cross-browser compatibility.