Use environment variable instead of string for API key

After working with the tests for awhile, I got a little bored with
remembering to remove my API key. Using an environment variable
makes it easier to make updates to the tests and makes sure no
one accidentally commits their key.

I also considered using a config file but there was too much
overhead.
Updated to reflect testing changes
Updated League to version 2.5

The new version removes several fields from the `LeagueItem` struct
that already appear in `League`. It also has new capabilities including

* support for Teams
* selecting up to 10 Summoners or Teams

With the addition of looking at Leagues by Team, I've added a function
that concatenates a slice of TeamIDs to a comma seperated string. It
works identically to `createSummonerIDString`.

Tests for the new Team interface in `league.go` have been added.
Updated Team to 2.4

The latest version of the Team API allows a client
to request up to 10 summoners at a time. There is also the ability
to search for a team based on their TeamID or up to 10 Teams.

I've also added a function to create comma seperated strings
based on TeamIDs. This is very similar to `createSummonerIDString`
except that it takes a `[]string` as an argument versus a `[]int64`.

A test has been added for `TeamByTeamID`.
WIP beginning of Match endpoint

Created all the datatypes
Merge pull request #31 from mattcan/env-var

Use environment variable instead of string for API key in tests
Merge pull request #32 from mattcan/league2.5

Updated League to version 2.5
Merge pull request #34 from mattcan/team2.4

Updated Team to 2.4
Fixed auto merging issues
Added match lookup method

Created a method to do match lookups in the Riot API. The API limits
only one match lookup and it is optional to download the entire
timeline of events.

Tests have been added as well.
Merge branch 'upstream-master' into match2.2

Merge all changes from TrevorSStone\goriot into match2.2
Slight changes for readability

Renamed the spell fields in `Participant` to match item fields
in `ParticipantStats`.

Moved the `matchID` to be the last argument in the event that
Riot allows searching for more than one in the future.
WIP Match history

Added a new data structure and fixed a reference to the wrong one
in `ParticipantIdentity`. The lookup method for isn't complete yet.

TODO:

* more tests
* expand number of champions allowed (arbitrary limit of 40)
* more clear way of creating comma seperated strings
Using a new way of creating url parameter strings

After discussing with @TrevorSStone on how best to proceed in #36,
I decided to go with two separate functions for creating parameter
strings. One is focussed on joining a `[]string` and the other
converts a `[]int64` to `[]string` and then joins. This is hidden
behind custom types and a `String()` interface.

I've used this function only in MatchHistory so far but the goal
is to eventually spread it to the other endpoints.
Updated tests for Match History
Merge pull request #35 from mattcan/match2.2

Match2.2 API