Run static analysis on CI, and fix errors

[hugovk]

Fixes the second part of #8.

• Run pyflakes on the CI to find possible coding errors:

$ pyflakes .
./setup.py:14: undefined name 'setuptools'
./gutenbergdammit/metadata.py:63: local variable 'found_names' is assigned to but never used
./gutenbergdammit/__init__.py:9: 'gutenbergdammit.textcleaner.TextCleaner' imported but unused
./gutenbergdammit/__init__.py:38: local variable 'e' is assigned to but never used
./gutenbergdammit/__init__.py:43: local variable 'e' is assigned to but never used
./gutenbergdammit/findbadzips.py:1: 'sys' imported but unused

• And fix them.

• And test installing as well.