* node.js: Add type guards for return types of util methods
* use object instead of Object in isObject typeguard
* use es2016 as target for Node 7 types
Node 7 supports almost 99% of ES2015 and 100% of ES2016. Hence, its safe
to use the target of es2016 in tsconfig.json
* remove typeguard from isObject function
* rename Symbol to symbol
* remove es2016 target in tsconfig.json