SendGrid: update to v3 send API
**SendGrid:** **[possibly-breaking]** Update SendGrid backend to newer Web API v3. This should be a transparent change for most projects. Exceptions: if you use SendGrid username/password auth, esp_extra with "x-smtpapi", or multiple Reply-To addresses, please review the [porting notes](http://anymail.readthedocs.io/en/latest/esps/sendgrid/#sendgrid-v3-upgrade).
Closes#28
In BasePayload, ensure any Django ugettext_lazy
(or similar) are converted to real strings before
handing off to ESP code. This resolves problems where
calling code expects it can use lazy strings "anywhere",
but non-Django code (requests, ESP packages) don't
always handle them correctly.
* Add utils helpers for lazy objects (is_lazy, force_non_lazy*)
* Add lazy object handling to utils.Attachment
* Add lazy object handling converters to BasePayload attr
processing where appropriate. (This ends up varying by
the expected attribute type.)
Fixes#34.
A message's `from_email` and each address in its `to`, `cc`, and `bcc` lists must contain exactly one email address. Previous code would silently ignore additional addresses, leading to unusual behavior. Now, raises new `AnymailInvalidAddress` exception.
Example: `from_email='Widgets, Inc. <widgets@example.com>'` is invalid: it needs double-quotes around the "Widgets, Inc." display-name portion. In earlier versions, this probably would have sent the message from something like "From: Widgets <@localhost>". Now, it will raise an exception.
**Potentially-breaking change:** If your code is using an unquoted display-name containing a comma in an email address, it will now raise an error. In earlier versions, this may have appeared to succeed, but was almost certainly not doing what you intended.
Fixes#44.
