fix imports after reorganization to package

I think that after reorganization to package it makes sense to make imports absolute - all submodules are no longer available directly, they are under `qtreactor` namespace. Imports at ghtTests and bin/gtrial didn't work; imports in modules located in `qtreactor` worked only because they were related to a current dir. To avoid such errors I've added `absolute_imports` future import to `qtreactor.*` modules.