Adds login/logout commands

[travishathaway]

Fixes: #2

[jezdez]

Noice! I love where this is going <3

[travishathaway]

@jezdez,

After a discussion with @kenodegard yesterday, we decided against the decorator pattern for the class methods. This would put too much burden on the subclass authors to remember to always put those there. Instead we now simply run them inside the base class' authenticate method.

@kenodegard,

I attempted to solely rely on lru_cache like you had mentioned for caching secrets. The problem I ran into is passing a dictionary is not allowed (i.e. the settings value). Because of that, I decided to stick with my initial method for caching but improving it so that it is easier for subclass writers.

Part of that was changing how secret retrieval works. There is now an abstract _fetch_secret method that must be defined, which is invoked by the fetch_method on the base class. The base class method handles the caching logic so that the child classes do not have to.

The API has now been improved so that fetching a secret for a channel is simpler. There is now get_secret method which accepts a channel_name parameter and attempts to find and return a value from its cache.

[travishathaway]

@kenodegard and @jezdez,

I believe this is ready for another round of review. On top of making the refactor changes requested, I have also rearranged the command namespace. conda login has become conda auth login and conda logout has become conda auth logout.

[jezdez]

Loving where this is going, only smaller API questions!