Rewrite test suite
Explicitly create pre/postprocessor functions during tests

Remove `dummyfn` fixture.
Check pre/postprocessors functions on init
Move fixtures to `conftest.py`
Update version number
Remove unused imports
Use application response class to create response object
Response rewriting works for each accepted mimetype
Move test, e.g. keep related tests nearby
Merge pull request #4 from vitalk/refine-test-suite

More clean and lean test suite.