Fix #40 - Hidden document viewer does not load pages in Firefox

There is a bug in Firefox where `window.getComputedStyle()`
returns null if called in a hidden (`display:none`) iframe
(https://bugzilla.mozilla.org/show_bug.cgi?id=548397), so we
fallback to a default value if this happens.
Merge pull request #45 from box/issue40

Fix #40 - Hidden document viewer does not load pages in Firefox