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.
1. Yes, Migrations use the data type of the property to determine the size of the sequence.
2. Not sure what happens when the sequence reaches the maximum value. I guess it is provider specific. SqlServer has CYCLE, NO CYLCE. The question still remains regardless of size. Plus, if a BIGINT sequence was used for an int column how is the existing code handling values out of range?
3. Maybe it can be avoided, I didn't give it much thought because it is more an EF metadata question. Migrations can do whatever we decide to support in metadata.