Envelope

- fix charset for base64 encoded content
- fix the default encoding while casting an attachment to string

- CHANGED:
* `.attach(filename=)` renamed to `.attach(name=)`
* `--attachment` renamed to `--attach`
* `.to()`, `.cc()`, `.bcc()`, `.from_()`, `.sender()`, `.reply_to()`, `.recipients()` now return an `Address` object(s), not `str`
- `.as_message` -> Message
- e-mail validation
- proper e-mail addresses handling: they can be in a string delimited by comma (or semicolon)
- text alternatives
- fix encoded headers
- fix attainable contents fetching
- embedding inline images
- loading parses attachments and decrypts
- contacts might be cleared (ex: to clear `To` header intuitively type `.to(False)`)
- `.attachments`, `--attachments [NAME]` -> read the list of the attachments
- preview returns nicely formatted headers

- fix: smtplib.SMTP_SSL handshake operation timed out caught
- if a line is longer than 1000 characters, makes the message be transferred safely by bytes (which would not break up DKIM)
- when "Content-Transfer-Encoding" is set to "base64" or "quo-pri" and reading message, it gets decoded (useful when loading EML files `cat file.eml | envelope --message`)
- preview method available from CLI

- fix: default object
- better EML files loading
- generic header manipulation (removing, adding multiple, mostly preserves order when loading)
- reply-to allows multiple addresses

- CHANGED:
* Envelope.__init__ parameters re-ordered
* `from envelope import Envelope` is the only right way to import, `import envelope` no more supported (slightly longer, however better suits Python naming conventions)
- fix: --attach-key flag in CLI did not work
- auto-import GPG key from file
- auto-encrypt GPG possibility
- S/MIME multiple recipients
- "sender" works as an alias for "from_" if both are not used; otherwise standard "Sender" header is included
- .date(date) method allows turn off automatic Date header adding
- fix: object is modified whenever a parameter changes (ex: if user changes subject multiple times), not at send time
- `.message` has new aliases: `.body` and `.text` to facilitate the programmer's workflow because when autocomplete

- smime dependency is optional – thus package can be installed directly without having swig library