Proposal for services/logs/edit and services/logs/delete

[following5]

This adds a method for editing logs, as proposed in #421. Log type, comment and date can be changed.

Not implemented in this first version:

• Changing from and to status log types like "Available". Users can submit a new log instead for correcting the geocache's status.
• Changing rating or recommendation. This is ridiculously complex; I would rather wait if someone needs it. E.g. c:geo so far does not support ratings and recommendations at all.
• Changing _needsmaintenance. This is tricky, and we may want to solve #548 first. Users may submit new a new log instead to indicate another maintenance state.

[following5]

Added proposal for services/logs/delete. So far it only allows deleting own logs. For delete-by-cacheowner, we need emails (#408).