Default value and Timestamp support

Initial support of DEFAULT value of columns.
Test classes and documentation will be added.

Sorry for having 2 features in 1 PR. 
The initial reason why I did this was that I wanted to have column with auto timestamp value written on insert. It is supported by Android and SQLite. So why don't this great library have it?

You can review the code. Change as you wish.
I send it earlier for you to review. If you are ok with the feature, I will improve it, document it and have test for it. 

Thank you.