verify that chosen is instanceof Chosen, fixes #1914
Merge pull request #1924 from harvesthq/gh1914-allow-init-with-data-attribute

Verify that chosen is instanceof Chosen, fixes #1914
Optimise PNG images with ImageOptim

| File                             | Saved           | Percentage |
| public/chosen-sprite.png         |   108/646 bytes |        16% |
| public/chosen-sprite@2x.png      |   134/872 bytes |        15% |
| public/docsupport/chosen.png     | 1958/3467 bytes |        56% |
| public/docsupport/oss-credit.png | 2309/6709 bytes |        34% |

(ImageOptim is available [here](http://imageoptim.com/)
Merge pull request #1951 from rjmunro/feature/optimise-images

Optimise PNG images with ImageOptim
Check for deltaY property of wheel event for compatibility with external plugins #1982
Merge pull request #1983 from efedorenko/wheel-event

Fixes scrolling with a mouse wheel in Firefox if jquery-mousewheel is used
Use content-box for text inputs

It's normally the default anyway but it's becoming very popular to set
* { box-sizing: content-box; } because it fixes so many annoying things.
Unfortunately, it can cause a problem when height and top or bottom
padding are set. In the case of this element, if border-box has been
set, we will only have 5px of vertical space left. So make sure it's
using content-box.
Merge pull request #2009 from okonomiyaki3000/text-input-box-sizing

Use content-box for text inputs
Fixing focus issue:

When disable_search flag is on, focus + hitting spacebar on the dropdown
causes the page to scroll and open the popup.
Adding preventDefault when the spacebar disables the scroll event to
happen.
Merge pull request #2046 from itayadler/master

Fixing focus issue:
Make all of Chosen use border-box
Merge pull request #2054 from harvesthq/chosen-box-sizing

Make all of Chosen use border-box