SendGrid: update to new v3 send API (#50)

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

anymail/utils.py

@@ -1,5 +1,6 @@
 import mimetypes
 from base64 import b64encode
+from collections import Mapping, MutableMapping
 from datetime import datetime
 from email.mime.base import MIMEBase
 from email.utils import formatdate, getaddresses, unquote
@@ -95,6 +96,20 @@ def getfirst(dct, keys, default=UNSET):
         return default
 
 
+def update_deep(dct, other):
+    """Merge (recursively) keys and values from dict other into dict dct
+
+    Works with dict-like objects: dct (and descendants) can be any MutableMapping,
+    and other can be any Mapping
+    """
+    for key, value in other.items():
+        if key in dct and isinstance(dct[key], MutableMapping) and isinstance(value, Mapping):
+            update_deep(dct[key], value)
+        else:
+            dct[key] = value
+    # (like dict.update(), no return value)
+
+
 def parse_one_addr(address):
     # This is email.utils.parseaddr, but without silently returning
     # partial content if there are commas or parens in the string: