function describe_column changed to get all column properties via column_properties in the row class

1. fix odbc_statement_backend::describe_column
fix handling for numeric columns -> return dt_integer, dt_long_long or
dt_double (merged from oracle backend)

2.  get all column properties during describe call
class column_properties enhanced to store informations about colSize,
decDigits and isNullable flag of a column
use a pointer of class describe_column for the describe_column function
call (be able to use the informations outside of SOCI)

Because i had to change all db-backends there is still an open task to add the informations from the backend to the column_properties class. Currently only ODBC and Oracle are adding all informations to the pointer. The other backends only add name and type.