TaskLogs fixes and cleanups.

- Fix negative offsets in FileTaskLogs and HdfsTaskLogs.
- Consolidate file offset code into LogUtils (currently used in two places).
- Clean up style for HdfsTaskLogs and related classes.
- Remove unused code in ForkingTaskRunner.
Merge pull request #769 from metamx/task-logs-fixes

TaskLogs fixes and cleanups.