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

anymail/backends/postmark.py

@@ -138,8 +138,10 @@ class PostmarkPayload(RequestsPayload):
     def init_payload(self):
         self.data = {}   # becomes json
 
-    def set_from_email(self, email):
-        self.data["From"] = email.address
+    def set_from_email_list(self, emails):
+        # Postmark accepts multiple From email addresses
+        # (though truncates to just the first, on their end, as of 4/2017)
+        self.data["From"] = ", ".join([email.address for email in emails])
 
     def set_recipients(self, recipient_type, emails):
         assert recipient_type in ["to", "cc", "bcc"]