Adding option for parallel to unload command

- Adds PARALLEL option to unload - [ Amazon Docs](http://docs.aws.amazon.com/redshift/latest/dg/r_UNLOAD.html?sc_ichannel=em&sc_icountry=global&sc_icampaigntype=launch&sc_icampaign=em_118617410&sc_idetail=em_1333791181&ref_=pe_411040_118617410#unload-parallel)
- Adds tests for parallel option

Signed-off-by: Jason Myers <jason@jasonamyers.com>