Prepare for usage of system-provided headers

[icedream]

Follow-up to and blocking #13.

We want to get rid of our own copy of the TS3 Plugin SDK and give the user an as convenient as possible option to compile against a user-installed copy of the TS3 Plugin SDK. Will have to look into how other cgo-based libraries solve something like this. Relying on pkg-config won't work here.

• Document compiling against pre-installed TS3 Plugin SDK headers
• Remove TS3 Plugin SDK from repository