Match 0 or 1 usercontent in RawGitHub URL Regular Expression

[rgbkrk]

No description provided.

[rgbkrk]

Only test failure remaining is

$ invoke test
........F.................
======================================================================
FAIL: test_github_blob_redirect (nbviewer.tests.test_github.GitHubTestCase)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/Users/kyle6475/code/nbviewer/nbviewer/tests/test_github.py", line 131, in test_github_blob_redirect
    self.assertEqual(r.status_code, 200)
AssertionError: 404 != 200
-------------------- >> begin captured logging << --------------------
requests.packages.urllib3.connectionpool: INFO: Starting new HTTP connection (1): localhost
requests.packages.urllib3.connectionpool: DEBUG: "GET /github/ipython/ipython/blob/rel-2.0.0/IPython HTTP/1.1" 404 1885
--------------------- >> end captured logging << ---------------------

----------------------------------------------------------------------
Ran 26 tests in 20.698s

FAILED (failures=1)