Zimbraweb

* Add is_parsable() to emlparsing

* Fixed an issue where the E-mail would not be formatted correctly if the Subject Header was too long.

* Fix an issue where Umlauts and other Unicode would not be converted correctly when the content-type was `text/plain`.

Broken documentation link in README

What's Changed
* Remove any hardcoded references to studgate.dhbw-mannheim.de or student.dhbw-mannheim.de Domain
* Adding verbose logging
* UTF-8 support
* Add attachment functionality
* Move `cookies` dict into `SessionData`
* Store Expiry Unix time of Auth Token cookie
* Add `ZimbraUser.logout()` method
* Add Sphinx Documentation and build it automatically
* Add package to PyPI: `pip install zimbraweb`
* Add `zimbraweb.emlparsing` module with `emlparsing.parse_eml()`
* Add `ZimbraUser.send_eml(emlstr: str)` method to send an eml file. Only `text/plain` is supported!

Changelog

- Users can only send from their own E-mail address. If they maliciously changed the `From:` header, sending will fail.
- UTF-8 support
- Attachment support
- Verbose Responses
- Logout function