Support fetching commit statuses for branches

[msvrtan]

Part of #23

[codecov-io]

Codecov Report

Merging #25 into master will decrease coverage by 2.84%.
The diff coverage is 21.87%.

@@             Coverage Diff              @@
##             master      #25      +/-   ##
============================================
- Coverage     46.53%   43.68%   -2.85%     
- Complexity      111      130      +19     
============================================
  Files            35       41       +6     
  Lines           490      554      +64     
============================================
+ Hits            228      242      +14     
- Misses          262      312      +50

Impacted Files	Coverage Δ	Complexity Δ
...Repository/Factory/Status/StatusCreatorFactory.php	0% <0%> (ø)	1 <1> (?)
src/V4/Raw/Repository/StatusApi.php	0% <0%> (ø)	2 <2> (?)
...pository/Factory/Status/ExternalServiceFactory.php	0% <0%> (ø)	6 <6> (?)
src/V4/Object/Repository/StatusObjectApi.php	0% <0%> (ø)	5 <5> (?)
...ect/Repository/Response/BranchStatusCollection.php	0% <0%> (ø)	3 <3> (?)
src/V4/Object/Repository/Factory/StatusFactory.php	100% <100%> (ø)	2 <2> (?)
... and 2 more

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