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.
About property Scheme, I set it to automatic property. Also I could do protected set but I have problem with constructor SqlScriptExecutor, which has ctor attribute scheme. So I leave it to public set.

About VersionTableName. I set it to "SchemaVersions", so  SqlContainer would have default journaling table name. If you think it is not necessary I will set also TableName to automatic property.