Support postgres & mysql in tests, fix quoting issue

Fixes #64 and reverts #60.

There is no One Quoting Syntax To Rule Them All. For a SET statement, postgres and mysql seems to disagree about how a column name should be quoted if it is quoted at all. Here, we go back to un-quoted column names. Though columns named "order" and other reserved words will cause errors, the solution works in all other cases.

Additionally, there is a commit introducing an easy way to test mysql and postgresql by setting an ENV variable.