Improve test suite

Currently, test suite is quite a overly complicated. It has two mode to run: "mock" and "couchdb". For the "mock" it mocks (obliviously!) HTTP request method and only test logic for some magically expected response. For "couchdb" mode it makes real requests to CouchDB instance and it makes sure that our imaginations of how things should work is close to real.

Initially, that was good idea: mock tests runs faster, require no service to run and, hell, why not? However, since those time it started to be bad idea because mocks hides real issues and keeping both modes to run the same test cases makes life harder.

Current plan is to completely move away from mock tests to integration ones. Despite all problems this will make sure that aiocouchdb really works with real service right.