Added support for sequences to migrations.

Added the following migration operations: MoveSequence, RenameSequence, AlterSequence.
Specialized DatabaseBuilder and ModelDiffer for SqlServer because the annotations for sequences are only available there (should we move to Relational?)
In SqlServerModelDiffer the sequences are matched based on the matching columns.
Added unit tests.
Because the sequence data type can be smallint, int, in which case ExecuteScalar returns Int16, Int32 objects that cannot be cast directly to long, unless I am missing something. Can do a switch on type code instead of Convert.ChangeType.