Invalid api urls don't fail gracefully

[santip]

Code such as the following:

api_client = ApiClient('INDEX_TANK_URL')

raise an ugly exception, and not on the client creation but on using the client to do a request, for instance:

  File "/home/edu/workspace/crawler/backend/user/app/lib/structured_indexer/sourcer.py", line 92, in submit
    response = index.add_documents(document_list)
  File "/usr/local/lib/python2.7/dist-packages/indextank/client.py", line 165, in add_documents
    return _request('PUT', self.__docs_url(), data=documents)
  File "/usr/local/lib/python2.7/dist-packages/indextank/client.py", line 395, in _request
    netloc_noauth = netloc.split('@')[1]
IndexError: list index out of range

Invalid urls should be caught on the constructor and we should probably support public urls too (the failing line of code suggests we don't :))