Move DBItest specification to DBI

[krlmlr]

I feel that the prose for the DBI specification is best suited in this package, because it is actually useful for DBI users. The prose is tightly connected to the specification code, but isolated from the actual testing logic, which can stay in DBItest for now.

One minor obstacle is that we don't want to introduce import dependencies, but the spec code requires testthat. We could make retrieving the actual spec code conditional on the presence of the testthat package.

@hadley: Do you have a strong opinion here?

[hadley]

Just that the prose explanations need to stay in this package.

[krlmlr]

"this package" = DBI?

[hadley]

Yes

[krlmlr]

Moving the definition of the DBI specs from DBItest to DBI looks too difficult, but DBI will contain a copy of the prose.