Add cancecled event to the date/time picker

[DimitarTodorov]

PR Checklist

• [x ] The PR title follows our guidelines: https://github.com/NativeScript/NativeScript/blob/master/CONTRIBUTING.md#commit-messages.
• [x ] There is an issue for the bug/feature this PR is for. To avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it.
• [x ] All existing tests are passing
• Tests for the changes are included

What is the current behavior?

There is no way to know if the user cancelled or confirmed.

What is the new behavior?

A "cancelled" event is emitted when the return value of the picker is null.

Fixes #41.