LeagueEndpoint update, new properties in championMastery and league

[kwiatkos0]

#621

#Deprecated

• GetLeaguePositionsAsync

#Added

• GetLeagueEntriesAsync
• GetLeagueEntriesBySummonerIdAsync
• GetGrandMasterLeagueAsync

#Fixed

• LeaguePosition.cs
• ChampionMastery.cs

[BenFradet]

Thanks 👍

[kirides]

This kinda makes #622 obsolete

[JanOuborny]

Thanks for your contribution 👍
Unfortunately, we have a merging problem now.
I propose that we merge #622 first, because it contains unit tests for the new methods and closer resembles the structure of the API. When it's done please rebase your branch onto develop and then we are going to merge the remaining changes.

[kwiatkos0]

Ok, I can also add unit tests and other things.