Add Streams, fixes #206

[mattbdean]

Some minor changes:

• There's a new interface called UniquelyIdentifable with one method: String getUniqueId().
• The upper bound for RedditIterable (and therefore Paginator) has been changed from Any (Object in Java) to UniquelyIdentifiable.
• RedditIterable has a new method: Stream stream(BackoffStrategy).

A Stream is just a subclass of Iterator. It continuously fetches new data as it arrives, or as quickly as its BackoffStrategy allows. Currently, the only BackoffStrategy is ConstantBackoffStrategy, which allows requests to be made at a certain rate.

Here's a practical example of using a Stream:

for (c in redditClient.subreddit("RocketLeague").comments().build().stream()) {
    // Do something with the newly-created comment
}

[codecov]

Codecov Report

Merging #249 into master will increase coverage by 78.99%.
The diff coverage is 74.6%.

@@              Coverage Diff              @@
##             master     #249       +/-   ##
=============================================
+ Coverage      0.53%   79.52%   +78.99%     
- Complexity        7      665      +658     
=============================================
  Files           120      124        +4     
  Lines          2237     2291       +54     
  Branches        282      291        +9     
=============================================
+ Hits             12     1822     +1810     
+ Misses         2224      295     -1929     
- Partials          1      174      +173

Impacted Files	Coverage Δ	Complexity Δ
...ib/src/main/java/net/dean/jraw/models/Comment.java	75% <0%> (+75%)	3 <0> (+3)	⬆️
...ib/src/main/java/net/dean/jraw/models/Message.java	66.66% <0%> (+66.66%)	2 <0> (+2)	⬆️
...src/main/java/net/dean/jraw/models/LiveThread.java	50% <0%> (+50%)	2 <0> (+2)	⬆️
...src/main/java/net/dean/jraw/models/LiveUpdate.java	80% <0%> (+80%)	2 <0> (+2)	⬆️
.../kotlin/net/dean/jraw/pagination/RedditIterable.kt	0% <0%> (ø)	0 <0> (?)
lib/src/main/java/net/dean/jraw/models/Trophy.java	66.66% <0%> (+66.66%)	2 <0> (+2)	⬆️
.../src/main/java/net/dean/jraw/models/Subreddit.java	51.72% <0%> (+51.72%)	8 <0> (+8)	⬆️
...c/main/java/net/dean/jraw/models/MoreChildren.java	60% <0%> (+60%)	5 <0> (+5)	⬆️
...src/main/java/net/dean/jraw/models/Submission.java	75% <0%> (+75%)	3 <0> (+3)	⬆️
...et/dean/jraw/pagination/ConstantBackoffStrategy.kt	100% <100%> (ø)	2 <2> (?)
... and 127 more

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