Refactoring DbUp and providers.

Based on #53. Done only changes in providers. 

1. Added methods in DbUp.StandardExtensions for calling scripts from file system in absolute or relative way.

2. Changed logging of exception in UpgradeEngine. Only return exception message, not whole Exception.ToString(). Some fixes regarding logging has been changed also in SqlSciptExecutor.

3. I have created classes IQueryProvider, QueryProviderBase, SqlServerQueryProvider) which contains all SQL statements used in source. Based on my previous experience this can be a life-saver, because you don't need to recheck all source code for SQL statements when something change in databases. All providers extend QueryProviderBase and have their own SQL statements written for some functionality.

4. All providers and their classes are in separate projects. Only exception is MsSql Server, which is still in base library DbUp. I think it would be also useful if classes used for MsSql server is moved to separate project.
I don't think SqlContainer should be exposed and then you can pass the table name into an overload of SqlLiteConnectionManagers ctor which in tern passes it through to the container?

Means we do not violate law of demeter here