Commands: Discover context types from migrations
Unladen swallow...

Add support for DefaultValue relational API

This one required a little more work than the others to allow a variety of different objects to be used. This is the last provider-specific relational API in the current batch so I can now work on removing the old code.
Added support for unique constraints.
GitHub Issue 839: Renamed UpdatedDatabase to ApplyMigrations and GenerateUpdateDatabaseSql to ScriptMigrations.