[B] Make player argument optional

**The Issue:**
I believe for /playSound the player parameter should be optional, and default to the sender.

**Justification for this PR:**
This PR was made to help improve command usage (ease of use), to remove unnecessary code and mainly to improve command speed/efficiency. 

For example, If i wanted to test a certain sound (to see if it matches my needs or not), (I have a list of all of the sound names) I would get the sound name, then paste it in in-game into the command syntax. This is good and dandy, until you start demoing a lot of sounds. 

I would really have two options when sampling a lot of sounds
1) Have to back arrow and remove the sound ID in chat yourself
2) Modify the list to have the command syntax in place (As in, put **/playsound** in front and **stuntguy3000** behind the sound ID.

I currently do the 1st option, as the 2nd option is rather slow, tedious and It really messes up the look of the document. 

**In summary: This PR is designed to make the command easier and simpler to use, as well as increasing speed and efficiency.**

**Description of changes:**
Each change is commented. Scroll down to view.

**JIRA Ticket:**
https://bukkit.atlassian.net/browse/BUKKIT-5374

**Testing Material**
Compiled changes from local fork and tested each argument.

Associated Bukkit PR: https://github.com/Bukkit/Bukkit/pull/1026