Add default values support ( https://github.com/jinzhu/gorm/issues/251 )

I added the default values support to gorm.

I updated the `README.md` and create the tests.
I tested it locally and works well.

When using the default values is required to fetch again the result of the insert/update statement (as is written into the `REAMDE.md`).
I think that's the only way we can handle these values, since in SQL we can specify default values that don't are strictly the same type of the field (think to the default values of "now()" for a time field or other expressions ).