Added offset and fetch functions to the SqlBuilder to allow pagination

[wbsantos]

I came across the need of making a pagination and I implemented the SQL Server way of doing it. It does work with postgree too since it is just ansi sql, but I think Oracle and MySQL don't have those clauses yet (although I wasn't able to test it against the latest version of those databases).

[ctwoodwa]

Maybe i'm missing something, but why not move this code into the SqlBuilder class and change it to public class SqlBuilder : Dapper.SqlBuilder

[wbsantos]

The SqlBuilder is a Dapper class, I'd have to make a pull request for the official Dapper repository to change it. Instead, I created a class that inherits from the Dapper.SqlBuilder. Maybe a partial class would suffice though