Fix global leaking

Hi! I've started with simple and short fixes.

  * Added IDEA project files to `.gitignore`.
  * Removed unused variable `prefixPersecond` and unused parameter `oldConfig`.
  * 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.

P.S. @mrtazz I have a huge desire (and also job need) to improve StatsD (coz it's awesome) and on weekend I found a lot of stuff I can contribute to your project. 

But I can't do much without your help and I asking you for strong cooperation.

I see a lot of unmerged pull requests and that's quite sad. Many of those are just oneliners, typos and docs fixes. Please, don't be so rude to your contributors.

I encourage you to review this PR asap so that I can start working on next one. Please, please, please. Let's make this world better together.

Thank you!