Use custom log file location if given in the args

At the moment, specifying a custom log file location in the command line parameters with [-logFile](http://docs.unity3d.com/Documentation/Manual/CommandLineArguments.html) breaks the console output for a build, because it always uses the default editor.log location for that Unity instance.

This change respects the custom log location for each job, and if I'm right this should let our Jenkins deployment build multiple jobs simultaneously using the same Unity instance. Currently, Jenkins reading from the same default log file messes up the console output.

I'm relatively new to the CI world so I may be wrong in my assumptions, but if not is this something that would be useful to have pulled into the main plugin? Your feedback would be appreciated :)