Improve 'To' and 'Reply-To' headers handling

[pataquets]

After feedback on #13 and testing 0086ec0:

• The resulting 'To:' field handling doesn't support multi-recipient field values (getaddresses() vs. parseaddr(). Attempted to fix that.
• Added a specific, stand-alone 'reply-to' field. This might enable in a future to use the 'to' property to just only the 'To:' header, but not sure, though.
• Added an examplo 'To:' header to a test email to easily demo a multi-recipient header (a tricky one, containing the field value separator, which is comma).

Needed/broken:

• Tests. I've seen where and how they're done.
• The to() property is currently ignoring the 'reply-to' (ouch!). How do I proceed? Merge 'to'+'reply-to' for compatibility or separate fields and break?
• Add test running to Dockerfile? (I wil grab them from .travis.yml).

I've paused work on #14 until I get more feedback and learn a bit more about the software, since they're related.
Disclaimer: Python is not my main language :)

[fedelemantuano]

Hi @pataquets:

• The resulting 'To:' field handling doesn't support multi-recipient... Ok I will test your suggestion and I will replace it.
• Added a specific, stand-alone 'reply-to' field. The fist version of mail-parser was in SpamScope, but now I will split to and reply-to and I will add cc. So for me it's ok.
• For unittest don't worry I will fix it.
• For Dockerfile I will do like in SpamScope. Travis will build and push images for me here