Use pre-commit

[pre-commit](https://pre-commit.com/) automatically runs tools like
linters and fixers to make development easier. This change adds a simple
pre-commit config file with an example trailing whitespace fixer and
applies it to the existing project.

Additionally, don't ignore the lib directory (an error introduced in #8).

.gitignore

@@ -26,7 +26,8 @@ dist/
 downloads/
 eggs/
 .eggs/
-lib/
+# pyShelf uses its own `lib` directory, don't ignore it.
+# lib/
 lib64/
 parts/
 sdist/