Add Bugdown tests for !avatar syntax

[timabbott]

I wrote tests for these, and the tests match what happens in production, but I get some really weird output running this in the test suite:

Running Bugdown test gravatar
Traceback (most recent call last):
  File "/home/tabbott/zulip/zerver/lib/test_runner.py", line 129, in run_test
    test_method()
  File "/home/tabbott/zulip/zerver/tests/test_bugdown.py", line 159, in test_bugdown_fixtures
    self.assertEqual(converted, test['expected_output'])
  File "/usr/lib/python2.7/unittest/case.py", line 513, in assertEqual
    assertion_func(first, second, msg=msg)
  File "/usr/lib/python2.7/unittest/case.py", line 924, in assertMultiLineEqual
    self.fail(self._formatMessage(msg, standardMsg))
  File "/usr/lib/python2.7/unittest/case.py", line 410, in fail
    raise self.failureException(msg)
AssertionError: u'<p><img alt="\x02klzzwxh:0000\x03" class="message_body_gravatar" src="/avatar/ [truncated]... != u'<p><img alt="[email protected]" class="message_body_gravatar" src="/avatar/ [truncated]...
- <p><img alt="�klzzwxh:0000�" class="message_body_gravatar" src="/avatar/�klzzwxh:0000�?s=30" title="�klzzwxh:0000�"></p>
+ <p><img alt="[email protected]" class="message_body_gravatar" src="/avatar/[email protected]?s=30" title="[email protected]"></p>

Here's the code:
https://github.com/timabbott/zulip/commits/markdown-bugs