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
Fixed the Gruntfile to rely on compass installed with bundler

The contributing documentation asks about installing compass through
Bundler, so the Gruntfile should expect it as well.
Version bump to 1.2.0
trigger ready event only after full instansiation, as documented
fixed typo
Merge pull request #2098 from liorch88/ready-event-fix

Trigger ready event only after full instansiation (as documented)
Added support for optgroup classes inheritance
Made the group classes support look more like the option classes support
Merge pull request #2106 from liorch88/optgroup-class-support

Added support for optgroup classes inheritance
Update chosen version in README
Merge pull request #2140 from ernsheong/master

Update chosen version in README