Cron Reschedule UI

Hi James,

This is my attempt at implementing cron reschedule UI for this plugin. I noticed that [there is a pull request for this already](https://github.com/jamescookie/quartz-monitor/pull/1) but wasn't quite fitting into the overall UI of the plugin. I tried to avoid this by adding a rescheduling icon to the 'Actions' menu.

In addition, I tried to use a popup box for the cron expression (as you said in the above mentioned pull request) if jQuery UI is available. The implementation is not very elegant due to having to deal with page refreshes (the popup disappears). Please let me know if there is any way I can improve it, alternatively just grab the first commit of this pull request (not quite sure if much is gained by having a popup...).

Thanks,
Zoran