Add convenience methods for IDbTransaction

At the moment, if you begin a transaction on an IDbConnection, in order to use that transaction in Dapper, one has to pass it in for every Dapper call made. Because an IDbTransaction implies an IDbConnection, it would be much more convenient if all the extension methods on IDbConnection that took a transaction parameter were also extension methods on IDbTransaction, using the transaction's Connection member for the connection. For example:

    public static int Execute(this IDbConnection cnn, string sql, object param = null, IDbTransaction transaction = null, int? commandTimeout = null, CommandType? commandType = null);

...would have extension method:

    public static int Execute(this IDbTransaction tran, string sql, object param = null, int? commandTimeout = null, CommandType? commandType = null) {
        return tran.Connection.Execute(sql, param, tran, commandTimeout, commandType);
    }

Start by locating the existing IDbConnection extension methods that accept an IDbTransaction. Add corresponding IDbTransaction convenience overloads using the transaction's Connection member, and verify that each delegates with the transaction supplied.