Support for multiple databases (internal only, not exposed through console)

This pull request adds support for migrating multiple related databases that have common versioning.
The database with version table is considered default and `Create` etc work as they did before, but the syntax below can be used to redirect certain changes to related databases.

**Syntax**
    
    InDatabase("Other")
        .Create.Table("Test").InSchema("test").WithColumn("Id").AsInt32();

**Notes on implementation**

1. This feature requires `MultiDatabaseMigrationProcessor` which wraps a set of other processors. This processor is currently not created when using console or MSBuild/NAnt tasks. The reason is that I do not need it at the moment (I use Runner manually), but I feel this pull request may be useful nevertheless.

2. Transaction support is a bit rough -- failing transaction on one DB may not revert transactions in others. I do not see a perfect solution for that, unless something custom is done with MSDTC (do all DBs support `TransactionScope` properly?)

3. New syntax makes old `IfDatabase` confusing. I would argue `IfDatabase` was already confusing and should be `IfEngine` or something like that. But it may also mean my extension should be renamed. I just did not have enough time for perfect naming.

4. Integration tests on MultiDB are only done in SQL Server 2008.

5. `IInDatabaseExpressionRoot` is a copy of `IIfDatabaseExpressionRoot`. Not sure if it should be reused instead (I noticed that it is not reused between `IfDatabase...` and similar code in `MigrationBase`).

6. I am not sure what level of XML comments is expected. I added some where similar classes had them or where it seemed useful. Let me know if more would be required.

7. For this, and for `InSchema` I feel supporting some kind of `using` may be a good idea. However this is probably not possible to implement without major changes.