Implement a checkbox function for vim-orgmode

Hi,

It's convenient to have a checkbox function for breaking down tasks.
So I have added checkbox functions and subtask information for vim-orgmode.

You can do checkbox toggling and autoupdate subtask information (like [1/3], [33%]), 
after toggling the checkbox.

I have provided some basic functions:
 1. Insert a new checkbox below/above: \<localleader>cn / \<localleader>cN
 2. Toggle a checkbox status: \<localleader>cc
 3. Update checkboxes and subtask information: \<localleader>c#

Some risks:
 - I add class "Checkbox" and reuse some code from class "Heading", I have extracted some common code from ""Checkbox" and "Heading" to a base class "DomObj".
 - The performance may not be good since I don't cached some information.
 - Unit testing is not enough.

Although I do lots of test on my machine, it could be some errors.
Please give me some feedbacks how to converge these changes.

Thanks,
Powen