Table base class for DBD::File & Co

Hi Tim, hi DBI developers,

inspired by the issues reported against DBD::DBM and SQL::Statement 
regarding Test-Database, I checked SQL::Statement, DBD::File and DBD::DBM to 
figure out where it fails.

I found several bugs which are fixed for now (even if I wouldn't tend to 
release this time). I'd like to get your opinion about following two changes:

1) Introduce a DBI::SQL::Nano::Table, deriving either from
    SQL::Eval::Table or DBI::SQL::Nano::Table_ - like
    DBI::SQL::Nano::Statement does.

    Derive DBD::File::Table from DBI::SQL::Nano::Table.

2) Add 2 additional tests (naming proposals) which test DBD::DBM
    and DBD::Gofer using SQL::Statement, if available.
    To be honest, I would let the 2 new tests use
    "$ENV{DBI_SQL_NANO} = 1" and remove this line from t/50dbm.t
    and t/85gofer.t
    The tests could be named t/zqs_$1

Following changes are planned to do before next DBI release:
- Synchronize DBD::DBM::Statement and DBD::DBM::Table with DBD::File
   classes
- allow case insensitive table -> file mapping for tables without ""
   (might need flags for the statement instance)


Jens