Implemented features

PlayerSelectors implements all the default minecraft selectors:
- @a - All players
- @p - Nearest player
- @e - Entities
- @r - Random player
- @s - Self/Sender
but also features a custom made one:
Since PocketMine implements a multi world system, you can easily select players from your current world using the "@w" selector!

Minecraft also supports specific search parameters such as the distance from the sender, the count of matched player/entities, ...
All the standard minecraft search params can be used with the syntax

@<selector name>[key=param,other=value]

EXCEPT the team & score params since the /scoreboard command isn't implemented into pocketmine yet
Thought, one new params comes with this plugin: the "lvl" param which is for searching in a specific level.

Have fun!