time_window priority in alert.py

[chaosong]

self.time_window 's value should be interval_to_graphite(options.get('time_window', self.reactor.options.get('time_window', self.interval))) in alert.py, or the time_window option is useless in the configuration's main block

[graearea]

This is a problem that we ran into. the configuration of time_window in the main block is completely useless and MUST be overwritten in each alert for it to work.