fix no more polling when last Host entry returned an error, add testing and more

[Apollon77]

• in updateDevices there was a race condition with the sand callbacks when the last host entry was not successfully returned - then the callback was not called and so the timeout was not set for the next cycle … means that the adapter did not checked anything anymore
• made sure by using a pollingTimer variable that only one timeout for the next cycle is running. there were cases where multiple timeouts were running (high load)
• change some small coding style things on jslint advice
• commented out dead code (return before code, so was never called at all)
• added some more error logging to get infos on errors to show error transparent in the log because they should be fixed, or ?!
• also start very first „updateAll“ call by a timeout because I had cases where it was started multiple times initially
• add basic adapter testing with Travis-CI and Appveyor (see https://ci.appveyor.com/project/Apollon77/iobroker-tr-064 and https://travis-ci.org/Apollon77/ioBroker.tr-064)
• add README note on supported node versions (4.x+)