added RawDefault ColumnType to map to RAW sql

I tried looking for an issue in assembla but I'm not even sure if you guys consider it an issue.

I am currently writing an app that uses slick and a lot of the primary id's are UUIDs and the DefaultValue option for me is practically useless. 

I would like UUID's to be generated on the Database (newsequentialid() (SQL Server) or uuid_generate_v1 (postgres) )  this way the indexes won't get too crazy.


Sadly you will not get any compile time checking for default values however this change would make it easier to point to default database functions.

I have also signed the CLA .