Add support for redshift.

Create a common class for redshift and postgres (since they are similar).
In the future postres should more to contrib as well.
Create both a Redshift target and an S3CopyTask
Use postgres as the Target for both Redshift and Postgres (only add an attribute use_db_timestamps). And rename postgreslike to rdbms
Merge pull request #276 from rantav/redshift

Add support for redshift.