fixes #437: fragile no-extend-native rule

I've also changed it to only affect the native constructors (not all
properties of the global object) and support ctor['prototype'] access.
fixes #431: no-unused-expressions should not flag statement level void
Merge pull request #438 from michaelficarra/GH-437

fixes #437: fragile no-extend-native rule
Merge pull request #439 from michaelficarra/GH-431

fixes #431: no-unused-expressions should not flag statement level void