Let the get_severity function return only the severity

Fixes the following Dialyzer warnings:
elarm_summary.erl:182: The call elarm_summary:update_summary_ack(Summary::any(),Severity::{_,_}) will never return since it differs in the 2nd argument from the success typing arguments: (#summary{},'critical' | 'indeterminate' | 'major' | 'minor' | 'warning')
elarm_summary.erl:346: Function update_summary_ack/2 has no local return
elarm_summary.erl:347: The call elarm_summary:decrement_count(Summary::any(),Severity::{_,_},'new') will never return since it differs in the 2nd argument from the success typing arguments: (#summary{},'critical' | 'indeterminate' | 'major' | 'minor' | 'warning','acknowledged' | 'new')
Merge pull request #24 from lucafavatella/fix-dialyzer

Fix some Dialyzer warnings