Tests: use `runResourceT` from other module

Conduit no longer re-exports this symbol.
Split out ImageSize into `imagesize-conduit` again

This code doesn't depend on GD, and is very useful on its own, so it
makes sense to have it in a separate package.
Depend on newly extracted `imagesize-conduit`
Merge pull request #2 from mbrock/master

Depend on resurrected imagesize-conduit package