Harmonize event emitters implementation

[ballinette]

main Kuzzle SDK prototype and websocket wrapper implementation both used their own event listeners.

Now, both Kuzzle and websocket wrapper inherits from Node build-in EventEmitter class (https://nodejs.org/api/events.html), with a polyfilled implementation for browser.

Breaking Changes

method addListener no longer returns an ID, to be compliant with NodeJS implementation ( https://nodejs.org/api/events.html#events_emitter_on_eventname_listener )
It now returns the Emitter object itself, so that it can be chained.

For each event, the listener callback function is used as the key in listeners list

Documentation

kuzzleio/documentation#172

[codecov-io]

Codecov Report

Merging #210 into 5.x will decrease coverage by 4.29%.
The diff coverage is 34.64%.

@@            Coverage Diff            @@
##              5.x     #210     +/-   ##
=========================================
- Coverage   99.36%   95.06%   -4.3%     
=========================================
  Files          16       17      +1     
  Lines        1733     1885    +152     
  Branches      465      511     +46     
=========================================
+ Hits         1722     1792     +70     
- Misses         11       93     +82

Impacted Files	Coverage Δ
src/Kuzzle.js	99.62% <100%> (+0.16%)	⬆️
src/networkWrapper/wrappers/websocket.js	100% <100%> (ø)	⬆️
src/eventEmitter/index.js	12.63% <12.63%> (ø)
src/Document.js	100% <0%> (ø)	⬆️
src/security/Security.js	100% <0%> (ø)	⬆️

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[ballinette]

See discussion on #197 that were approved, but merged on the wrong branch