Fixing use of "bigint" and "decimal" for pure integer types.

See silverstripe#14 for more info.

"bigint" doesn't work for PHP 32-bit, as the integer could get too big
for PHP to be able to interpret the value as an integer type. In that
case it would try to approximate it as a float, but that could lead
to a bad assumption for identity columns.

For compatibility, stick with standard "int" type which has a maximum
value of 2147483647.

Note that MySQLAdapter already has this maximum, so changing it to
this keeps this database adapter consistent with MySQL.

Another PR for backporting to 3.0/3.1 compatible version was raised here: https://github.com/silverstripe/silverstripe-mssql/pull/16