logviewer tries to determine log directory via logback configuration, falls back to storm.home/logs
use File methods to construct paths
add logviewer.appender.name config
logviewer locates log directory via appender named in storm config

exits on startup if it can't find the appender
throw RuntimeException and remove unnecessary System/exit
clearer arg name
Merge pull request #705 from strongh/logview-logback

logviewer tries to determine log directory via logback configuration