Fix up BG Parsing in Events, PP Calculation & Add in simple hit judgements

[kpei]

This is a change made on top of #69

Fixed bg parsing if no offset was specified
Added back some basic class methods for beatmap because performance_points method was erroring
Simple hit judgement stuff mostly so I can calculate UR. Don't have to implement it, it's just a wrapper I made around Replay.hits method.

[Liam-DeVoe]

I'm going to close this in favor of #69 and #103 (bg without offset parsing is fixed in the former). The addition of a judgment class is definitely useful, but it's with too many other changes in this PR. Feel free to create a new PR with just the judgment class changes if you want.