release 0.6.0

Since 0.5.0 we fixed the following issues:

• #66 (closed): (refactor) learner.tell(x, None) might be renamed to learner.tell_pending(x)
• #92 (closed): DeprecationWarning: sorted_dict.iloc is deprecated. Use SortedDict.keys() instead.
• #94 (closed): Learner1D breaks if right bound is added before the left bound
• #95 (closed): Learner1D's bound check algo in self.ask doesn't take self.data or self.pending_points
• #96 (closed): Learner1D fails when function returns a list instead of a numpy.array
• #97 (closed): Learner1D fails when a point (x, None) is added when x already exists
• #98 (closed): Learner1D.ask breaks when adding points in some order
• #99 (closed): Learner1D doesn't correctly set the interpolated loss when a point is added
• #101 (closed): How should learners handle data that is outside of the domain
• #102 (closed): No tests for the 'BalancingLearner'
• #105 (closed): LearnerND fails for BalancingLearner test
• #108 (closed): (BalancingLearner) loss is cached incorrectly
• #109 (closed): Learner2D suggests same point twice

and merged the following Merge Requests:

• !93 (merged): add a release guide
• !94 (merged): add runner.max_retries
• !95 (merged): 1D: fix the rare case where the right boundary point exists before the left bound
• !96 (merged): More efficient 'tell_many'
• !97 (merged): Fix #97 (closed) and #98 (closed)
• !98 (merged): Resolve "DeprecationWarning: sorted_dict.iloc is deprecated. Use SortedDict.keys() instead."
• !99 (merged): Resolve "Learner1D's bound check algo in self.ask doesn't take self.data or self.pending_points"
• !100 (merged): Resolve "Learner1D doesn't correctly set the interpolated loss when a point is added"
• !101 (merged): Resolve "Learner1D fails when function returns a list instead of a numpy.array"
• !102 (merged): introduce 'runner.retries' and 'runner.raise_if_retries_exceeded'
• !103 (merged): 2D: rename 'learner._interp' to 'learner.pending_points' as in other learners
• !104 (merged): Make the AverageLearner only return new points ...
• !105 (merged): move specific tests for a particular learner to separate files
• !107 (merged): Introduce 'tell_pending' which replaces 'tell(x, None)'
• !112 (merged): Resolve "LearnerND fails for BalancingLearner test"
• !113 (merged): Resolve "(BalancingLearner) loss is cached incorrectly"
• !114 (merged): update release guide to add a 'dev' tag on top of regular tags
• !115 (merged): Resolve "How should learners handle data that is outside of the domain"
• !116 (merged): 2D: fix #109 (closed)

New features

• add learner.tell_pending which replaces learner.tell(x, None)
• add error-handling with runner.retries and runner.raise_if_retries_exceeded
• make learner.pending_points and runner.pending_points public API
• rename learner.ask(n, add_data) -> learner.ask(n, tell_pending)
• added the overhead method to the BlockingRunner

I think we should release 0.6.0 since we introduce some new features. @jbweston and @anton-akhmerov do you have a different opinion?