Fix #311. Add regex to check for field assignment
Update fix for E731

Thanks to @mypalmike for thinking of the extra cases
Merge pull request #312 from sigmavirus24/bug/311

Fix #311. Add regex to check for field assignment
Replace the IDENTIFIER_REGEX with the isidentifier function
Fix too long line introducing E501 error
Fix definition of isidentifier in Python 2
Add toplevel build dir to git ignore file
Catch import errors thrown by os.path.expanduser(); issue #297

* This can happen particularly with AppEngine when importing pwd
Update changelog for issue #297
Fix false positive E711/E712; issue #336
Fix false positive E713; issue #330