Updated to include single entry Riot API endpoint

Riot added the ability to only look up a summoner's entry in their
league instead of having to load all of the people in their league.
New function supports that feature.

Test written for new feature.
Added ability to view league stats for challenger

Riot recently added the ability to look at Challenger tier so I
updated here to support the three different types: `RANKED_SOLO_5x5`,
`RANKED_TEAM_3x3`, and `RANKED_TEAM_5x5`.

Test case was added.
Forgot to fmt
Merge pull request #22 from mattcan/league-2.3

League 2.3 - Challenger and single entry by summoner Fixes #20
Added helper strings for all regions.

Also fixed a minor typo in a comment