Add support for SQL Server

* I only did run the tests for SQL Server, didn't want to trouble with installing PostgreSQL and MySQL
* Please take an extra good look at commit: 553320967d129fb9b5b408a116abfa46f00e35b7
* I added SQL Server to the tests that should be run by default, I don't know if you want that. The SQL Server adapter depends on having FreeTDS installed
* I took the liberty of adding the "pry" gem as a development dependency
* I took the liberty of creating a subclass of `ForeignKeyDefinition` to handle SQL Server differences. Instead of explicitly creating calling `new`on `ForeignKeyDefinition` I added a new method, `foreign_key_definition_class` on `AbstractAdapter`, which returns the class to use. The SQL Server adapter then overrides this method and returns a subclass. Part of commit: 559ff3faa525b2cbd0b6287dde5b7451f278050a