Fixing `length` check.

`hasOwnProperty('length')` is not supposed to work on an `NodeList` as it's an inherited property.
Fixing `window.devicePixelRatio` stubbing.

It was not overridden in Gecko. The code was altered to accept an optional context, possibly not containing the expected property.
Merge pull request #72 from oncletom/fix-firefox-testing

[READY] Gecko Compatibility
`window.addEventListener` -> `var addEvent`
Changing tests to see if chai can be possibly compatible with IE8
Page offset generator and tests
Fixing IE/W3C eventlistener
JSHint compliance
Converted tests to expect.js

It is compatible with Mocha, Karma and mostly, IE6!
It then requires a very few changes from chai.