Weather alerts cached too long

[we7u]

Weather alerts are cached with a time-stamp in several files. If Xastir is shut down for a period of time and then started, it may bring display weather alerts that should have already expired. We should check the timestamps, load only if they're within a specific window of time, plus actively delete expired alerts from the cache files.
~/.xastir/logs/wx_alert.log
~/.xastir/logs/wx_alert.log.1
~/.xastir/logs/wx_alert.log.2