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.
So are we saying that people must override these methods? If so, why are they not abstract?