Simplify, document, and fix the API code.

We used to create endpoints with Client._register.

Now we now have explicit methods for the endpoints.

This allows us to add docstrings and stricter mypy annotations.

This fix also introduces a call_endpoint() method that avoids
the need for manually building urls with API_VERSTRING when you
know the URL pattern of the endpoint you want to hit (and when
the API doesn't have a convenient wrapper).

I fixed a bug with create_users where it now uses PUT instead
of POST.

I also removed client.export(), which was just broken.

I had to change recent-messages and zulip-export, which were
using client.do_api_query and Client._register.

Now it's easier to just call client.call_endpoint() for
situations where our API doesn't have convenient wrappers,
so that's what I did with those scripts.

examples/recent-messages

@@ -35,7 +35,7 @@ Example: recent-messages --count=101 --user=username@example.com --api-key=a0b1c
 
 You can omit --user and --api-key arguments if you have a properly set up ~/.zuliprc
 """
-sys.path.append(os.path.join(os.path.dirname(__file__), '..'))
+sys.path.insert(0, os.path.join(os.path.dirname(__file__), '..'))
 import zulip
 
 parser = optparse.OptionParser(usage=usage)
@@ -45,7 +45,7 @@ parser.add_option_group(zulip.generate_option_group(parser))
 
 client = zulip.init_from_options(options)
 
-req = {
+request = {
     'narrow': [["stream", "Denmark"]],
     'num_before': options.count,
     'num_after': 0,
@@ -53,7 +53,12 @@ req = {
     'apply_markdown': False
 }
 
-old_messages = client.do_api_query(req, zulip.API_VERSTRING + 'messages', method='GET')
+old_messages = client.call_endpoint(
+    url='messages',
+    method='GET',
+    request=request,
+)
+
 if 'messages' in old_messages:
     for message in old_messages['messages']:
         print(json.dumps(message, indent=4))