Use ExecutorService intead of ThreadPoolExecutor in AsyncHttpClient

Changed the setter to use ExecutorService because that's its threadPool
member type and Executors.newFixedThreadPool() returns an ExecutorService
(this method is referenced from the setThreadPoll() javadoc)
Merge pull request #543 from erikghonyan/master

Thanks @erikghonyan, this is indeed better.
Updated build (both Travis and Gradle configuration)
Merge remote-tracking branch 'origin/master'
Add "has" method, to check parameter existence.

Can be useful since the real parameters map is protected.
Use java boolean
Allow developers to provide their own implementation for AsyncHttpRequest.
Merge pull request #560 from ebuildy/patch-1

Add "has" method, to check parameter existence.
Merge pull request #564 from fineswap/AsyncHttpRequest

Allow developers to provide their own implementation for AsyncHttpRequest.
Updated Javadocs to get valid build output
Updated build configuration for latest Android SDK distribution package
updated Travis configuration
support org.xml.sax.helpers.DefaultHandler for SAXParser
Added additional default binary response handled content types #573
Handling correctly null URLs in calls, related to #572
Fixed #537
Provided way to clear all headers Closing #581
Javadoc issue fixed [skip ci]
Added contribution rules [ci skip]
Wrong file name [ci skip]