TraceLogger

Since MS is actively forcing Trace logging in 4.5 - this logger looks like 'must have' ;)
And also I've 'compacted' loggers code a bit - there was some unneeded copy-paste. I cant even call this 'refactoring' - too small changes. But now it must be a bit easier to create yur own LoggerFactory (if needed).

With trace logger you can redirect log events to whatever logger you want:
```xml
  <system.diagnostics>
		<sources>
			<source name="CqlSharp.CqlConnection.Open" switchValue="All">
				<listeners>
					<add name="nlog" />
					<remove name="Default" />
				</listeners>
			</source>
		</sources>
		<sharedListeners>
			<add name="nlog" type="NLog.NLogTraceListener, NLog" />
		</sharedListeners>
  </system.diagnostics>
```
Without any additional code for LoggerFactories.