(refactor) learner.tell(x, None) might be renamed to learner.tell_pending(x)

This issue is a result of a discussion we had here: !68 (comment 15204)

I suggested splitting learner.tell(x, value) into two functions,

1. one with value=None: make this learner.tell_pending(x) as this method to me seems more intuitive
2. one with value!=None: keep this the same as it was before

This change will be a breaking change to the api, however we could accept the old implementation for a certain amount of time (e.g. a few months) and raise a DeprecationWarning in the meantime.

While we're at it, we may also change some other functions, @jbweston also suggested having learner.peek(n) instead of learner.ask(n, add_data=False). This falls in a similar category.

Other naming suggestions than learner.tell_pending and learner.peek are also welcome.

Lastly I would also really enjoy not passing real=True/False to any method, in the beginning I had a hard time understanding the code for this reason, and most functions had an if statement to check if real was True or False, so in essence it is just two functions merged into one.