Add additional postgres and redshift time grains

[itajaja]

redshift docs: http://docs.aws.amazon.com/redshift/latest/dg/r_Dateparts_for_datetime_functions.html
postgres docs: http://www.postgresql.org/docs/9.1/static/functions-datetime.html

Not sure if it's sensible to test this.
"I tried it and it works" gave 100% coverage for this :D

[landscape-bot]

Code quality remained the same when pulling 0d44bd9 on itajaja:additional-pg-grains into 88c9516 on airbnb:master.

[coveralls]

Coverage remained the same at 80.869% when pulling 0d44bd9 on itajaja:additional-pg-grains into 88c9516 on airbnb:master.

[mistercrunch]

Thanks for the PR!