Makes single backstroke deleting the default behavior.

I think we should consider ripping out the double backstroke
supporting code entirely.
Update docs to reflect default change on single_backstroke_delete
Properly handle defaults on single_backstroke_delete

`@options.single_backstroke_delete || true` is _always_ true.
Replace `javascript:void(0)` with `#`

Fixes:
- issues with content security policy (#1215)
- onbeforeunload in IE (#403)

We don't need to add a preventDefault because all clicks on
.chzn-container already have a preventDefault
Spell pressing correctly.
Merge pull request #1375 from harvesthq/single-backstroke-delete-default

single_backstroke_delete defaults to true
No href at all!
Update stackoverflow link to search for chosen posts
Merge pull request #1383 from harvesthq/stack_overflow

Update stackoverflow link to search for chosen posts
Added @koenpunt to README
Merge pull request #1384 from koenpunt/update-readme

Added @koenpunt to README
No need to expose SelectParser and AbstractChosen when joining before compile
Merge pull request #1389 from koenpunt/object-context

No longer expose classes to window context
When choosing a result in single select mode, set the current value
to false (If it exists, of course).

Fixes #1391
Disable Chosen on iOS devices (iPad, iPod, iPhone)
Only tag mobile as not supported browser
case insensitive
Added anchor tags to documation headers
Added footer to option page
Merge pull request #1392 from harvesthq/single-select-should-unset-current-value

Single select value fix