Previously, it was entirely plausible for us to remove something from
options.ignore overzealously. This is more confined logic and much more akin
to what I was intending the behaviour to be.

Extra include eggs

Add `.eggs` and `*.egg` to `EXTRA_EXCLUDE`

This is because these are usually downloaded by setuptools and they're usually other people's code so you don't want to check them.

```
[marca@marca-mac2 requests-unixsocket]$ flake8
./.eggs/pbr-0.10.0-py2.7.egg/pbr/packaging.py:740:17: E265 block comment should start with '# '
./pbr-0.10.0-py3.4.egg/pbr/packaging.py:740:17: E265 block comment should start with '# '
```

See merge request !10

Exit Flake8Command.run() cleanly if style checks pass

When using distutils/setuptools, it is common to perform several commands in sequence. For example: `python setup.py flake8 bdist_egg`. When `run()` exits by raising a `SystemExit` exception, it causes the entire chain to break - meaning flake8 cannot be used in combination with any other command.

This change simply alters the behavior to exit normally if there were no style violations found. Otherwise, it will exit immediately, as before, with a non-zero exit code.

See merge request !8

In order to better prevent regressions (such as related to concurrency),
Add a integration test framework to simulate running flake8 with
arguments.

Subprocesses write to stdout. They must flush their output before the
main process exits to avoid losing output or confusing the calling
program.

I have seen people who place the comment at the bottom of the file and it
would be backwards incompatible if we did anything but preserve it as a search
instead of match