Relative path resolving fix. Fixes #829

Currently `configPath` was not copied because it was implemented as `property` :disappointed: 

I did not change the nature of `configPath`, but fixed its flow. Except `additionalRules` fix, I also fixed `errorFilter` tests, because they tested for the wrong behaviour.
`.jscs.json` to `.jscsrc`, since we would need to deprecate (i assume) that name anyway