Make ItemCheckbox a controlled component

[Ulmanb]

Hey, nice work. I used ItemCheckbox and encountered an issue when I wanted the check value to be determined by the application state - when the relevant state changed the ItemCheckbox would re-render but without changing the display or taking into account the new "checked" property value.

Changes:

• Made ItemCheckbox a controlled component so its value will be managed by the containing component(which solved my issue).
• Removed the default property since it had no good use anymore.
• Minor updates to the readme