Clean-up: Do not manipulate `sys.path` even is runn without installation.

Changing `sys.path` may have other side-effects. Instead of this try to
import `pep8ify` from the parent directory if `pep8ify.fixes` fail to
import.
Clean-up: Use `raise SystemExit` intead of `sys.exit`.
Clean-up: Use lib2to3.main.main qualified.

Calling just `main(...)` is a bit irritating.
Clean-up: Use double quotes for a string containing a single quote.

This makes emacs properly recognize the string.
Add fixtures for testing mixed indents (2 and 4 spaces).
More useful output on test-error: show diff between expected and result.

Instead of using lib2to3 for showing the diffs between _in file and result,
use difflib and display the diffs between _out file (which is the expected
result) and real result.
Clean-up: Globally define number of spaces to replace tabs.
Clean-up: Remove useless class attributes.
Display useful description about fixture name (take it from filename).
Minor clean-up: Rename a misleading local var into a not correct name.
Add fixtures for testing multi-dedents.
Fix: Correctly handle lines dedenting more then one level at once.
Clean-up: Replace list if indents by a indent-level counter.

The content of the list-elements is now unused, so a simple
counter suffices.
Add assertion: only the first dedent must have a prefix.
Add more fixtures for testing multi-dedents.
Fix: Dedent did dedent preceding dedent node.

Need to set `current_line_dedent` node on every change if line-number.

This includes another test fixture.
Refactor intention of comments into a separate function.
Small code clean-up to enhance readability.
Move special case for newline from fix_indent_prefix to transform_newline.
Fix intention of comments.

This change includes a test-fixture.