api: Use the console_scripts entry point to point to zulip-send.

Instead of using the `scripts` keyword, we now use the
`console_scripts` entry point to point to the zulip-send script
to be installed. This is what the Python Packaging User Guide
recommends for better cross-platform compatibility.

setup.py

@@ -57,7 +57,11 @@ package_info = dict(
                   "examples/subscribe",
                   "examples/unsubscribe",
                   ])] + list(recur_expand('share/zulip', 'integrations/')),
-    scripts=["bin/zulip-send"],
+    entry_points={
+        'console_scripts': [
+            'zulip-send=zulip.send:main',
+        ],
+    },
 )  # type: Dict[str, Any]
 
 setuptools_info = dict(