Support the `COPY` command

Lifted from https://github.com/jeremyevans/sequel/blob/master/lib/sequel/adapters/postgres.rb#L375-L405 where most of the exception handling is now done in https://github.com/sprsquish/vertica/pull/18.

I plan to get the stuff in the vertica gem out instead of this gemfile hack which only temporarily works for development before merging. 

Thoughts?