Shedule type should be plugin

[Thierry Bressure]

The scheduler type available inside TracCronPlugin should be plugable. So every one could write a scheduler as a regular Trac compoenent (a.k.a plugin) that TracCronPlugin look for by Extension mechanism.

This will let someone else to write a real cron string parser scheduler or a funny scheduler that fire task based on weather condition (why not, calling a web service, not to hard to do....)

[Thierry Bressure]

r9325