Split stdout and stderr in different files - #103

@qconer Hi, this looks great. Can you convert all your tabs to spaces before I merge this PR? It's throwing off the diffs.

@qconer Hi, this looks great. Can you convert all your tabs to spaces before I merge this PR? It's throwing off the diffs.

@hellogerard Hi. Already.

This PR also closing an issue #79

This change breaks Jobby on Windows. Any jobs which have an output file specified result in the following error:
The process cannot access the file because it is being used by another process

I did some debugging and found that the culprit is the exec line in BackgroundJob. When an output file is specified and no stderr is specified, Windows tries to write to the same file for both with the following syntax:
exec("$useSudo $command 1>> \"$stdoutLogfile\" 2>> \"$stderrLogfile\"", $dummy, $retval);

However, this is not valid syntax for Windows. The correct syntax is shown here.