This way we can avoid code duplication.
Also, this change makes tests faster due to ignoring the sdist step
and adds Python 3.2, 3.4 and PyPy 3 to build matrix.
- The "--no-site-packages" option is default now
- "pip install -e" is basically equivalent to "python setup.py develop"
- Delete also dist/ and MANIFEST
- Delete all *.py[co] files (and __pycache__ directories on Python 3)