add remove a single subscriber

[vincentvvv]

There was no method to remove a single subscriber.

Subscribe will return a subscriber which can then be used to call removeSubscriber by the Observable.

https://compass-tech.atlassian.net/browse/MOBILE-4799

[codecov]

Codecov Report

Merging #38 into master will increase coverage by 0.53%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #38      +/-   ##
==========================================
+ Coverage   90.59%   91.12%   +0.53%     
==========================================
  Files          10       10              
  Lines         117      124       +7     
  Branches       12       13       +1     
==========================================
+ Hits          106      113       +7     
  Misses          7        7              
  Partials        4        4

Impacted Files	Coverage Δ
...l-kotlin/src/main/java/com/compass/snail/Replay.kt	100% <100%> (ø)	⬆️
...ail-kotlin/src/main/java/com/compass/snail/Just.kt	100% <100%> (ø)	⬆️
...tlin/src/main/java/com/compass/snail/Observable.kt	88.5% <100%> (+0.41%)	⬆️
...ail-kotlin/src/main/java/com/compass/snail/Fail.kt	100% <100%> (ø)	⬆️

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[russellbstephens]

blocking - lets also look at a possible autodispose solution