Prevent text selection on Chosen

This is especially nice when users click and drag to select.

Fixes #292
Makes single backstroke deleting the default behavior.

I think we should consider ripping out the double backstroke
supporting code entirely.
Added events example and descriptions to documentation
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.
Updated events documentation
Added info about params sent with events
Merge pull request #1374 from harvesthq/user-select-none

Prevent text selection on Chosen
Merge pull request #1376 from harvesthq/add_events_to_documentation

Added events example and descriptions to documentation
Spell pressing correctly.
Merge pull request #1375 from harvesthq/single-backstroke-delete-default

single_backstroke_delete defaults to true