Enable flake8 tests

Ran both by Travis and locally. There's some setup required, README updated.
A few important Flake8 checks are still disabled, we're going to enable them as
soon as the stubs are fixed and we can reliably run Flake8 locally with Python
3.6.

runtests.sh

@@ -2,6 +2,4 @@
 
 ./tests/mypy_test.py
 ./tests/pytype_test.py
-
-# FIXME: Enable when errors listed in `.flake8` are fixed in all stub files.
-# find . -name "*.pyi" | xargs -s 1024 flake8 --builtins=StandardError,apply,basestring,buffer,cmp,coerce,execfile,file,intern,long,raw_input,reduce,reload,unichr,unicode,xrange
+./tests/flake8_test.py