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
Enable request to local files

When a request to local files using XMLHttpRequest succeeds, it returns a status equal to zero (0) and
with responseText attribute filled. These changes make viewer.js return an ajaxSuccess() when those conditions have been meet.
Merge branch 'local-files-request' of github.com:appglu/viewer.js -- closes #47

* 'local-files-request' of github.com:appglu/viewer.js:
  Enable request to local files
Fix insertCSS/appendCSSRule/deleteCSSRule tests in Firefox
Move all test libraries to npm dependencies and upgrade sinon, qunit, sinon-qunit, and jquery
Merge pull request #48 from box/upgrate-test-framework

Upgrade test framework
Fix jQuery dependency version for IE < 9 support
Fix #49 - Centering broken if Bootstrap css loaded
Merge pull request #50 from box/issue49

Fix #49 - Centering broken if Bootstrap css loaded
Remove duplicate CSS rules