New access method via qxtLog macro

[mschnee]

Old code follows the form:

qxtLog->info() << "Some Message" << "Some other message";
qxtLog->info("A single message");

We're moving away from the "Log Level" syntax and towards a "Log Target" syntax:

qxtLog("info") << "Some Message" << "Some other Message";
qxtLog("info","Some Message");

1. qxtLog is currently a macro that resolves to a pointer to the singleton. It should instead resolve to a reference to the singleton.
2. Implement the necessary operators: operator()(), operator->(), and the necessary overrides to directly log or to return a QxtLogStream.