Build fixes and clean-up

* reflect Python 3.7 release
* fix `pycodestyle` errors
* update `pycodestyle` config
* move `pytest` and `pycodestyle` config to `setup.cfg`
* add `make pycodestyle`
* add `make coveralls`
* etc.

.gitignore

@@ -2,12 +2,12 @@
 .idea/
 __pycache__/
 dist/
-httpie.egg-info/
 build/
 *.egg-info
 .cache/
-.tox
+.tox/
 .coverage
 *.pyc
 *.egg
 htmlcov
+.pytest_cache/