auth only available in login command

[0xbase12]

Connected to #372

[loomis]

I think that more of the uses of the HTTPClient need to be fixed. Here's a stacktrace from the ss-module-get command:

pypi> ss-module-get apps
Traceback (most recent call last):
  File "/Users/loomis/Library/Python/2.7/bin/ss-module-get", line 73, in <module>
    MainProgram()
  File "/Users/loomis/Library/Python/2.7/bin/ss-module-get", line 37, in __init__
    super(MainProgram, self).__init__(argv)
  File "/Users/loomis/Library/Python/2.7/lib/python/site-packages/slipstream/command/CommandBase.py", line 89, in __init__
    self._callAndHandleErrorsForCommands(self.doWork.__name__)
  File "/Users/loomis/Library/Python/2.7/lib/python/site-packages/slipstream/command/CommandBase.py", line 122, in _callAndHandleErrorsForCommands
    res = self.__class__.__dict__[methodName](self, *args, **kw)
  File "/Users/loomis/Library/Python/2.7/bin/ss-module-get", line 59, in doWork
    client = HttpClient(self.options.username, self.options.password)
AttributeError: Values instance has no attribute 'username'

[loomis]

Just removing the username/password references seems to work (adfa9f7), but there may be more subtle changes needed.