timeline: changed dimensioning from content-box to border-box (fix for issue 128)

these changes to `timeline.css` allow for a more precise rendering of - especially - ranges in modern browsers (ie8+, fx2+, gc10+, sf5.1+). it is a follow up for issue #128.

all elements with...

1. a stated `width` and/ or `height`, AND
2. a stated `padding` and/ or `border`

...in the final html (via the system stylesheet, and/ or the library stylesheet (timeline.css), and/ or directly via javascript), received two extra declarations in `timeline.css`:

    -moz-box-sizing: border-box;
    box-sizing: border-box;

note that any mention or reference to the dimensions of these elements will now refer to the _content including padding and borders_, instead of _content_ only.

i tested these changes for events box, dot and range – with and without navigation – locally at:

* box – http://tests.vangorkum.com/javascript/library/chap-links/border-box/timeline.test.box.html,
* dot – http://tests.vangorkum.com/javascript/library/chap-links/border-box/timeline.test.dot.html,
* range – http://tests.vangorkum.com/javascript/library/chap-links/border-box/timeline.test.range.html.

i did not copy these changes to the jquery themeroller stylesheet (timeline-theme.css).