Mailgun, SparkPost: support multiple from_email addresses

[RFC-5322 allows](https://tools.ietf.org/html/rfc5322#section-3.6.2)
multiple addresses in the From header.

Django's SMTP backend supports this, as a single comma-separated
string (*not* a list of strings like the recipient params):

    from_email='one@example.com, two@example.com'
    to=['one@example.com', 'two@example.com']

Both Mailgun and SparkPost support multiple From addresses
(and Postmark accepts them, though truncates to the first one
on their end). For compatibility with Django -- and because
Anymail attempts to support all ESP features -- Anymail now
allows multiple From addresses, too, for ESPs that support it.

Note: as a practical matter, deliverability with multiple
From addresses is pretty bad. (Google outright rejects them.)

This change also reworks Anymail's internal ParsedEmail object,
and approach to parsing addresses, for better consistency with
Django's SMTP backend and improved error messaging.

In particular, Django (and now Anymail) allows multiple email
addresses in a single recipient string:

    to=['one@example.com', 'two@example.com, three@example.com']
    len(to) == 2  # but there will be three recipients

Fixes #60

tests/test_postmark_integration.py

@@ -43,7 +43,8 @@ class PostmarkBackendIntegrationTests(SimpleTestCase, AnymailTestMixin):
         message = AnymailMessage(
             subject="Anymail all-options integration test",
             body="This is the text body",
-            from_email="Test From <from@example.com>",
+            # Postmark accepts multiple from_email addresses, but truncates to the first on their end
+            from_email="Test From <from@example.com>, also-from@example.com",
             to=["to1@example.com", "Recipient 2 <to2@example.com>"],
             cc=["cc1@example.com", "Copy 2 <cc2@example.com>"],
             bcc=["bcc1@example.com", "Blind Copy 2 <bcc2@example.com>"],