Fix for issue #58: clean up Reportal mail temp directory

I accidentally removed the clean-up code in this commit - https://github.com/azkaban/azkaban-plugins/commit/906b938d2dca4c3fc8f25eb18cab2418980ca47a

I've added it back here and also fixed some indentation issues and made some variable names clearer.

Internal JIRA: HADOOP-4512