Changes

SLF4J Logger rewrite part2 (#492)
SLF4J stage 2 (final).
- Added SLF4J Logger (fallback) and JDA internal Factory.
- Added SLF4J to the README.
- Replaced logs using variables with lazily built logs using SLF4J
variable injection.
- Combined multi-logs into a single log message (exceptions!)
 Also did:
- Added try/catch in onTextMessage to print full JSON package when an
error occurs outside of handling
- Added handling for CHANNEL_UPDATE for a not existing category
 (commit: e9399b7)