Add keyboard handlers. ENTER & SPACE will cause the current action or, if no button focused (which should not happen - onblur handlers attempt to constrain focus to the buttons), will confirm the modal. ESC will cancel the modal, if a cancel button is visible, otherwise will do nothing. TAB will cycle through the buttons.
Merge branch 'master' into add-keyboard

Conflicts:
	example/index.html
	lib/sweet-alert.js
Merge branch 'master' into add-keyboard

Conflicts:
	lib/sweet-alert.css
Improve my keyboard logic according to some cool bits from the keyboard-shortcuts branch, plus handle leaving and re-entering the window better.
Fix focus states for buttons
Cleanup for release
Merge branch 'nixta-add-keyboard'