Mail-deduplicate

- Reject mails without `Message-ID` headers if `--message-id` option is
active.
- Add a class to holds global config.
- Print more debug info in unittests when CLI produce tracebacks.
- Always check for package metadata in Travis CI jobs.
- Fix package's long description against PyPi rendering constraints.
- Add link to full changelog in package's long description.

- Refactor and redefine all removal strategies. Closes {issue}`40`.
- Add a new `-t`/`--time-source` CLI parameter to point to the canonical
source of a mail's timestamp.
- Rename `-s`/`--show-diffs` parameter to `-d`/`--show-diff`.
- Rename `-D`/`--diff-threshold` parameter to
`-C`/`--content-threshold`.
- Add new `-s` shorthand to `--strategy` parameter.
- Re-implement diff threshold options. Closes {issue}`43`.
- Re-implement statistics. Closes {issue}`42`.
- Normalize and canonicalize all paths to deduplicate on the fly mails
pointing to the same file.
- Build documentation via Sphinx.
- Add `test` and `develop` dependencies.
- Move coverage config to `setup.cfg`.
- Replace `pep8` package by `pycodestyle`.
- Enforce `pycodestyle` checks in Travis CI jobs.
- Remove popularity badge: PyPI download counters are broken and no
longer displayed.
- Test production of packages in Travis CI jobs.
- Move development and packaging documentation to Sphinx. Closes {issue}`22`.
- Make wheels generated under Python 2 environnment available for Python
3 too.
- Let unittests generates their own fake and temporary maildirs.
- Print CLI output in unittests.
- Use generic factory to produce mail fixtures in unittests.
- Only show latest changes in the long description of the package
instead of the full changelog.

- User-friendly progress bar.
- Decrease memory usage on large datasets. Closes {issue}`19`, {issue}`8` and {issue}`3`.
- Attempt several encodings when parsing message body. Closes {issue}`32`.
- Fixed comparison issue in Python 3. Closes {issue}`34`.
- Add a set of basic deduplication unittests.

- Use logger to output messages to the user.
- Activate tests on Python 3.3, PyPy and PyPy3.
- Fix date parsing. See {issue}`33`.
- Fix decoding of unicode header value. Closes {issue}`24` and {issue}`32`.

- Add Python 3.4 and 3.5 support. Closes {issue}`30`.
- Add default `isort` configuration.
- Remove hackish default encoding forcing in main code.

- Fix `AttributeError` on message instance. Closes {issue}`28`.
- Various fixes of references to internal variables introduced by a
switch to more Pythonic classes in 1.0.0.
- Remove `Message-ID` from the list of default canonical headers.