New operator .group_by

[bittrance]

Useful operator for e.g. session tracking and similar cases. Part of the push to cover most of RxJS 5.

[codecov-io]

Codecov Report

Merging #66 into master will increase coverage by 0.17%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
+ Coverage   99.44%   99.61%   +0.17%     
==========================================
  Files          76       77       +1     
  Lines        4492     4680     +188     
==========================================
+ Hits         4467     4662     +195     
+ Misses         25       18       -7

Impacted Files	Coverage Δ
lib/rx/linq/observable/group_by.rb	100% <100%> (ø)
lib/rx/testing/async_testing.rb	93.93% <0%> (-6.07%)	⬇️
lib/rx/testing/test_scheduler.rb	100% <0%> (ø)	⬆️
lib/rx/concurrency/current_thread_scheduler.rb	100% <0%> (ø)	⬆️
lib/rx/concurrency/threaded_enumerator.rb	100% <0%> (ø)	⬆️
lib/rx/operators/multiple.rb	100% <0%> (+0.19%)	⬆️
lib/rx/testing/reactive_test.rb	100% <0%> (+1.72%)	⬆️
lib/rx/concurrency/immediate_scheduler.rb	100% <0%> (+12.96%)	⬆️

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[jake-007]

Just wanted to say thank you, using group_by right now in our project.