Added support to cross-fade some battle music when encountering enemies.

As client game map info parameters were loaded after the game mode was activated
due my recent change about loading the level only once the server tells which to load,
I had to fix the game mode to activate only after the client game map level loading.

This fixed the calm music not played anymore when starting the game.

I also made the server tell the specific client when attacked or attacking because
the attack and damage taking logic was only done on server-side.

Fixes #23
Fixed remote client game start

The client/server events would not be processed without that fix.
Made it so a server message is only sent to the client when starting or stopping the battle music.

As discussed with @hwoarangmy, I moved the battle music timing logic into the serve game map,
and the battle music timer into the player class, as it is a player thing, as righteously said
by hoarangmy.

I also normalized the battle music to avoid hearing glitches in the sound as reported by Akien.
Oops, fixed a wrong comment.
Small fixes based on @hwoarangmy's input. :)
Typo fix
Merge pull request #140 from Bertram25/development

Added support to cross-fade some battle music when encountering enemies.