GUAC-911: Add log levels. Refactor guac_client_log_*() into guac_client_log().
GUAC-911: Reduce available log levels to ERROR, WARNING, INFO, and DEBUG.
Merge branch 'master' into GUAC-844

Conflicts:
	src/guacd/client.c
	src/guacd/daemon.c
	src/guacd/log.h
	src/libguac/guacamole/client-types.h
	src/libguac/guacamole/client.h
	src/libguac/user-handlers.c
	src/protocols/vnc/client.c