Added IDEA project files to .gitignore
Removed unused variable
Fix `prefixStats` global variable leaking

`prefixStats` is leaked through stats.js and was in global scope of event emitter without failing graphite backend by pure accident.
Now it is read from config.
Removed unused parameter
Merge pull request #437 from anton-rudeshko/fix-global-leaking

Fix global leaking of prefixStats