Abstracted DateInput and added Key handlers

Supported keys:
- ArrowUp: increase date type (year/month/day)
- ArrowDown: decrease date type (year/month/day)
- Enter: close date-picker
Removed unneeded semicolon
Processed @markijbema's comments
Merge pull request #34 from Hacker0x01/airplane/key-handlers

Abstracted DateInput and added Key handlers