Support for Microsoft SQL Server style "SELECT TOP 5..." version of the limit clause

This quick patch will allow Idiorm to generate correct limit clauses for SQL Server. If it detects the driver is "mssql" or "sqlsrv", or alternately the "limit_clause_style" configuration option is set, it will generate a statement like "SELECT TOP 5 * FROM widgets" instead of the more traditional "SELECT * FROM widgets LIMIT 5".

This is by no means a complete implementation of a SQL generator for MSSQL - but it's a start and allows basic functionality. Notably missing is support for the "OFFSET" clause, which is not handled at all (that is, it will generate invalid SQL and the driver will throw an exception in that case).

Also included are updates to the test framework that uses a different mock for the DummyPDO class. By using a new class instead of inheriting from the PDO base class, it's easy to fake things like the driver it claims to be using. This required a little bit more code and less flexibility in the mock, but it does allow us to test the different SQL generation for MSSQL (which includes the differing quote character which does not appear to have been tested previously.