bots: Create Link Shortener Bot.

Create Link Shortener Bot using the goo.gl Link Shortening API.

Link Shortener Bot can be mentioned in a conversation, and it will
respond with shortened, goo.gl links for every URL in the message.

For example,

 > @link_shortener_bot @johnsmith Check out this file:
 > https://github.com/zulip/python-zulip-api/blob/master/zulip_bots/

and Link Shortener Bot would respond

 > https://github.com/zulip/python-zulip-api/blob/master/zulip_bots/:
 > https://goo.gl/Mt5z3c

In order to use Link Shortener Bot, an API key for goo.gl must be set
in `link_shortener.conf` in the `link_shortener` folder.

zulip_bots/setup.py

@@ -52,6 +52,7 @@ setuptools_info = dict(
         'html2text',  # for bots/define
         'BeautifulSoup4',  # for bots/googlesearch
         'lxml',  # for bots/googlesearch
+        'requests'  # for bots/link_shortener
     ],
 )