fix issume #162, some convert improvement with issue #160, fix bug: wrong data_type return at oracle_statement_backend::describe_column

1. fix issue #162
  support ORM bulk into
  test with test_orm_bulk(), commontests.h 

2. some convert improvement with issue #160
  test with test_convert();
  
  no we can convert to any integral type, like:
  row& r = ...;
  r.get< int >(0);
  r.get< float >(1);
  r.get< short >(2);

3.fix bug: wrong data_type return at oracle_statement_backend::describe_column

>create table T(t1 number);
>for column t1, oracle_statement_backend::describe_column(...) return dt_integer, should return dt_double


4.add two tests: 
    test_orm_bulk();
    test_convert();