Re-allow for relative paths for exclude

Previously, all testing was done from the directory in which the
configuration file lived, so this bug went unnoticed. However, if you
run Flake8 against its own source from a directory above, you would
notice that the patterns in the exclude config value in tox.ini were
ignored. This is because we (like any reasonable person) are using
relative paths. The path is relative, however, to the directory in
which the configuration file was located. So we keep track of which
directory that is and use that to normalize the paths in the config
file.

Yes, there is an unrelated change to our tox.ini in this commit as
well. ;-)

Closes #194