Envelope

- 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

- CHANGED:
* `encrypt-file` changed to `encrypt-path` (to match the `encrypt_path` parameter of the `encrypt` method)
* parameter swap from `.encrypt(sign=None, key=True)` to `.encrypt(key=True, sign=None)` due to S/MIME that does not take the key from the GPG keyring but needs the certificate to be specified every time.
- S\MIME
* signing (`M2Crypto` instead of `smime` package)
* insert subject while encrypting
- unit tests + travis build status
- SMTP
* if SMTP/INI file is given as a relative path and not found at CWD, we try program directory (useful when importing Envelope as a library from another program)
* TLS security supported
* if security not defined, determined by port
- bash completion
- --quiet flag

- fix launching with no flags (bare message erroneously tried to be GPG-signed by default)

- attachment file not found caught
- output attached to the complex example in the README
- fix launched in a loop