Modal container rewrite

[walterbm]

See issue #18.

Full rewrite/refactor of the ModalContainer component. Component is now fully compatible with React 14 (removed direct references to getDOMNode()) and has fully functioning life-cycle events (beforeOpen & afterOpen, beforeConfirm & afterConfirm, and beforeClose & afterClose).

Also includes integration tests through cypress and a new ModalContainer playground page.

Breaking changes:

• rename the propsbeforeSave, afterSave, and saveText to beforeConfirm, afterConfirm, and confirmText
  • using "save" implies that the component will be responsible for the data but in practice these are only life-cycle hooks. Also, many modal use-cases do not require saving data and it could be confusing to contradict these cases.
• remove the target prop. It was unused and did not affect functionality.

[Kyle-Mendes]

Before accepting we need to:

• [x] Clean up the playground page (styling & keep open toggle)
• [x] Tests for the lifeCycle events