Leaderboards functions

[yancouto]

Added many leaderboard related functions. (They are being used on MarvInc :D)

Added functions:

• init (no longer called automatically)
• findLeaderboard
• findOrCreateLeaderboard
• getLeaderboard(DisplayType/SortMethod/EntryCount/Name)
• uploadLeaderboardScore
• downloadLeaderboardEntries
• activateGameOverlay
• activateGameOverlayToWebPage
• getFriendPersonaName

Callbacks:

• onUserStatsReceived
• onUserStatsStored
• onUserAchievementStored

Other changes:

• Changed onGameOverlayActivatedCallback to make it more similar to the original API
• Removed outdated README stuff. In the next PR (WIP) I'll add proper documentation.

[Kazuo256]

If the PR is not ready yet, it's better to put a "WIP" somewhere in its title ;)

[yancouto]

It's done now :P I had just noticed I forgot some stuff while I was writing the documentation so I tried to sneak some changes before the review :)

[yancouto]

Instead of always using brackets, I added a clang-format rule for putting if statements without brackets on the same line, and I'm using that for small if statements like

if (condition) return;

This way, there's no confusion because of indentation (and also there wouldn't be normally, since clang-format will indent everything properly).

For large single statements I'm still using braces, since it does look clearer.

if (condition) {
    this_is_a_very_large_statement(with_a_bunch_of_args);
}

---

Also, I didn't add the error messages since the API itself says they're kind of useless, but I'll add them later, once I do some refactoring.

[henriquegemignani]

if (condition) return;

This way, there's no confusion because of indentation (and also there wouldn't be normally, since clang-format will indent everything properly).

I prefer to have all return statements very clearly visible and not obscured by an if statement.

[yancouto]

Done.