Monthly adjust

This pull request is for two changes:
- Add fixed monthly adjustment capability. Friends of mine tell me this works great for them, and they had mixed result with weatherunderground (I am quite happy for now). opensprinkler_pi also has this capability. My twist is that one may decide on a different set of monthly adjustments per zone, or just define a default, or any mix of these two modes. Sorry, no documentation yet. See config-template.json for an example ("sand" refers to the type of soil, which impacts the watering adjustment).
- Fixed a few refresh issues with weather and calendar, optimize the event display (don't request all events periodically: just check the latest one) to avoid growing CPU overhead as the history gets longer, add more information for monitoring adjustments and cleanup the web client code in the process. These changes are to stabilize the software that is currently running.