fixes issue #232

This fixes https://github.com/mozilla-services/heka/issues/232

FileOutput will now try to create the parent directory of a logfile if it doesn't exist.  Write permission is verified on startup - failure to have write permission will cause Heka to abort with an error message.

Dashboard will run some write permission checks on startup causing Heka to abort if those permissions don't exist.

Fixed a small typo in the configuration documentation for the `working_directory` option for the DashboardOutput.