Introduce flake8 configuration.

This is currently disabled due to the high number of errors that need fixing.
I'll be committing fixes for them and updating the numbers in .flake8 until we
can enable linting for all files by default.

.flake8

@@ -0,0 +1,54 @@
+# Some PEP8 deviations are considered irrelevant to stub files:
+# (error counts as of 2016-12-19)
+# 17381 E704 multiple statements on one line (def)
+# 11840 E301 expected 1 blank line
+#  7467 E302 expected 2 blank lines
+#  1772 E501 line too long
+#  1487 F401 imported but unused
+#  1248 E701 multiple statements on one line (colon)
+#   427 F811 redefinition
+#   356 E305 expected 2 blank lines
+
+[flake8]
+ignore = F401, F811, E301, E302, E305, E501, E701, E704
+
+# Errors that we need to fix before enabling flake8 by default:
+#  3097 E261 at least 2 spaces before inline comment
+#   921 F821 undefined name
+#   221 E128 continuation line under-indented for visual indent
+#   213 E231 missing whitespace after , or :
+#    85 E266 too many leading ### in a block comment
+#    59 E265 block comment should start with #
+#    52 E402 module-level import not at top of file
+#    48 E225 missing whitespace around operator
+#    44 E127 continuation line over-indented for visual indent
+#    37 E126 continuation line over-indented for hanging indent
+#    30 E124 closing bracket does not match visual indentation
+#    29 W391 blank line at end of file
+#    28 E116 unexpected indentation (comment)
+#    26 F403 from * import used
+#    20 E101 mixed spaces and tabs
+#    19 W191 tabs
+#    14 E251 unexpected spaces around keyword argument '='
+#    14 E203 whitespace before
+#    10 W291 trailing whitespace
+#     8 B006 mutable argument defaults
+#     7 E999 invalid syntax
+#     7 E111 indentation is not a multiple of four
+#     5 W293 blank line with whitespace
+#     5 F405 name undefined or from * imports
+#     5 E123 closing bracket does not match indentation of opening bracket
+#     4 E262 inline comment should start with #
+#     4 E121 continuation line under-indented for hanging indent
+#     4 E114 indentation is not a multiple of four
+#     3 E303 too many blank lines
+#     3 E125 continuation line with same indent as next logical line
+#     2 W292 no newline at end of file
+#     2 E241 multiple spaces after ,
+#     2 E131 continuation line unaligned for hanging indent
+#     2 B303 __metaclass__ use on Python 3
+#     1 E401 multiple imports on one line
+#     1 E202 whitespace before )
+#
+# Those error codes are disabled by default in pycodestyle:
+# E121, E123, E126, E241

runtests.sh

@@ -2,3 +2,6 @@
 
 ./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