API v2 support

[nicholastay]

API v2 support should be added into this library. - https://osu.ppy.sh/docs/index.html

For now, I reckon we shouldn't implement undocumented endpoints, as well as endpoints whose responses aren't finalised yet. It should probably be a mode to use v2, so that the two can even be used in tandem via two instances of nodesu if desired.

I'm not sure about the OAuth endpoints that requires users - will this be useful, how should we expose this to a library user (e.g. allowing access token to be passed in per call), or just simply not support this and remain a public API wrapper only (i.e. 'public' scope endpoints only implemented). Input from current library users about this would be appreciated.

Chat bots will probably be outside the scope of this project, though.