Fixes #20

* adds _MANIFEST.in_ files to all distributions
* adds `include_package_data=True` to all _setup.py_ files.
* registers test resources in the _MANIFEST.in_ files
* removes `sys.path.append(...)` hacks
* replaces `__file__` and `__path__` references with
  `pkg_resources` API calls in all tests
* removes `pkg_resources.require(...)` from all tests