Further support for countdown timer

[mgmart]

These changes should fix #72

Added an additional initialiser which allows to set the countdown timer
to an initial countdown interval.

Changed picker view configuration regarding the existence of countdown
timer.

[skywinder]

Thanks! I'll review it soon!

[skywinder]

Thanks.
I decide to not add additional init method because I found, that it can cause potential bugs in this realisation.
It's better to use common init methods, and set countdownDuration parameter the same way (like minimumDate, maximumDate, etc.)

Also, I add this improvements:

• Update example with CountDownUsage and add Swift examples for all type of DatePickers
• Update property name according oiginal name of this property
• Remove unused init function.
• Add CountDownTimer support to blocks.
• Remove redundant countDownDuration init

[mgmart]

Thank you.
You're right about the initializer.