Adding default values for columns in models

[jwoertink]

Fixes #390

This adds the ability to set your default values for columns in your models. If your columns require default values, you'll still have to do it in the migrations as well.

If this gets merged in, then the next step will be to update the SchemaEnforcer to ensure that the migration and model both have the same default value set.

[paulcsmith]

Nice! Will this cause issues with the SchemaEnforcer raising when it shouldn't? I'm guessing it just doesn't check the default right? If so then I think this is good to go since there is no downside to merging!

If it causes issues with SchemaEnforcer then we can still merge but we just need to be careful not to release a patch version until the SchemaEnforcer is updated

[jwoertink]

Ok, I added a spec for the SchemaEnforcer to make sure it doesn't just randomly bork. The other specs are all passing, so I don't think this will affect anything currently. Then in another PR I'd like to update that SchemaEnforcer to account for the default values better.

[jwoertink]

Also, just learned this neat little trick:

This will return all of the columns that have a default value in your database.

select table_name, column_name, column_default from information_schema.COLUMNS where table_schema = 'public' and column_default is not null;

This could be a good use for the schema enforcer

[paulcsmith]

Everything looks good! And nice find on the default values thing!