Detect N806 errors in multiple assignment

Add .idea and .pytest_cache to gitignore

.idea is a folder used in PyCharm IDE. Some other PyCQA projects like
pylink and pydocstyle also have it ignored.

This will, at least partly, resolve issue #29.

fix ignore bug in class names

Detect class methods defined within compound statements

Remove spurious 'xxx' (fixes #58)

Fix #45

Only check ast.Name for their id in metaclass check

This should avoid attribute errors for checking the ``id`` attribute on
classes that don't have a basic inheritance declaration.

Closes gh-53

Also restructure to ensure we're testing the right stuff.

Move "__ in function name" check to new N807 code

Use the project's canonical PyCQA URL

This check appears controversial (#9) because PEP8 doesn't mention this
as an official naming rule. This change moves it to its own error code
so that it can be disabled separately.

Consider all metaclass methods to be class methods

It's a widely used (but perhaps not official?) convention to consider a
metaclass's methods to be class methods and to use `cls` as the first
argument (rather than `self`).

Use the simple heuristic of "class inherits from `type`" to identify
metaclasses and mark all of their methods as CLASSMETHOD.

Bump version and add release notes for 0.5.0

Add configurable list of classmethod / staticmethod decorators

- Added support for parsing CLI-style flake8-config parameters in tests
  using flake8_polyfill (has a stable API which flake8>=3 lacks)
- Removed support for multiple `codes` in testcases (unused)
- Clean up incorrect usage of default which caused redundant work in
  parse_options for ignore_names

Fixes #38

Show offending symbol as part of the  message

Fixes github issue #31

Drop Python 2.6 support

make some regexps raw strings

Correct N804/N805 for __init_subclass__

According to pep 0487, the `__init_subclass__` method is implicitly
considered a classmethod. Thus it should be considered as an exception
of N804/N805 (first argument should be named self/cls). This is already
implemented for `__new__` so extend using the same logic.

to avoid DeprecationWarnings with Python 3.6

Travis CI's pypy3 is very old and breaks when we try to use setuptools
with it. Python 3.6 is something we should start testing on.

Include license