Support for EFMigration and Database creation

After following this guide for EntityFramework http://fxjr.blogspot.com/2013/06/npgsql-code-first-entity-framework-431.html I realized there is no support for automatic creation of database and tables... So i created support for DatabaseExist, DatabaseDelete and DatabaseCreate in NpgsqlServices.cs. Also added NpgsqlMigrationSqlGenerator in constructor so it's loaded by dependency injector...

Wrote NpgsqlMigrationSqlGenerator which does not handle Procedures and switching type from int to serial(doesn't add default value to new sequence...) plus I'm not sure how well Indexes work...

I will soon start using this migration on project I'm starting so I'm making this pull request to make people aware of it if someone wants to play with it. You could also merge it it effects only #if ENTITIES6...

My point is... its not complete yet...