Envelope

- 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

- set signing to "auto" for signing if there is a key matching to the "from" header
- preview method
- recipients method
- read subject(), message()
- load method

- choose mime subtype (html or plain) + conditional conversion of line breaks to <br> 4
- experimental load function to create e-mail object from text templates
- fix: possibility to include own "Content-Type" into headers

- fix: Subject tried to be re-set when no GPG neither S/MIME used