add support for AWS Organizations

endpoints covers so far:
- create_organization
- describe_organization
- create_account
- describe_account
- list_accounts

all tests passing.
could use some advise from maintaners.

moto/organizations/urls.py

@@ -0,0 +1,10 @@
+from __future__ import unicode_literals
+from .responses import OrganizationsResponse
+
+url_bases = [
+    "https?://organizations.(.+).amazonaws.com",
+]
+
+url_paths = {
+    '{0}/$': OrganizationsResponse.dispatch,
+}