Added a 'client' option for Postgres Bulk Load to support hosted environments.

We are using this with `activewarehouse-etl` on Heroku. Heroku does not support Postgres bulk copy directly from a file, but they do support streaming the data from STDIN. 

This change adds a `:client` option to the `bulk_load` method so that CSV data can be sent from the client to the server via STDIN instead of having the db directly access the file.

If the `:client` option is not specified, then `bulk_load` will behave as it did before.