Added support for DB2

These changes add support for DB2 within fluentmigrator. This code was developed for internal use on an as/400 system, so it probably will not account for all versions of DB2.

Some alter/drop statements may require an additional step as a side effect of how DB2 for iSeries handles potential data loss. There is more info on this in the included readme file.

All tests were passed on a local dev system running v5r4. Let me know if I've missed anything.

Thanks!