Add support for container other than the body

Passing options.containerElement in as an option will generate the loader inside that element instead of the body.
Set pos absolute if containerElement is set

.. otherwise, you can't tell the difference with position:fixed!
Update nprogress.css
Add support for container other than body
Revert "Set pos absolute if containerElement is set"

This reverts commit 4948e52812aa898c25ee61f40da5f30a3d8888fd.
Update tests to use mocha-jsdom.
Update 'contributors' link

It was pointing to another repo.
Merge pull request #99 from rictorres/patch-1

Update 'contributors' link on Readme.md
Merge remote-tracking branch 'remotes/rstacruz-original/master'

Conflicts:
	package.json
Merge remote-tracking branch 'remotes/psyked/pull-request'

Conflicts:
	nprogress.css
	nprogress.js
Merge remote-tracking branch 'remotes/psyked/master'

Conflicts:
	nprogress.css
	nprogress.js
- corrected error in last merges

- augmented the index.html demo page to showcase the NProgress component better: the interactive buttons remain visible while they now show the ability of NProgress rendering inside an alternative parent DOM element.

- adjust the index.html example to show the addEventListener support for onDone and onDoneBegin: instead of accepting a single function (callback), these can invoke a collection of registered listeners instead, making for a far more flexible hooking into and use of nprogress.

- fixed the CSS/LESS to correctly facilitate the 'render in another parent than the BODY tag itself' functionality: all positioning is `absolute`, rather than `fixed`. Only when the NProgress component is applied to the BODY tag, does the CSS position change to `fixed` to simplify that way of using it (we don't have to bother with any margins or borders the userland CSS may have set on the BODY tag that way).
having fun while showing a visual horizontal separation between the top and bottom segments of the demo page: the top segment will be overlapped by the interactive NProgress instance while the bottom segments contains its controls.