#73 Self disabling strategies

[xtian]

Sometimes strategies disable themselves because prerequisites for running are missing.
This should not happen because prerequisites might have been checked against the wrong village if the user interacted while the nanny was running.
For example, when the market is not found the ResourceSender is disabled, but the market might not be found when the user changes village on the browser.
Another case is when the user modifies the configuration. If a strategy had been previously disabled (like an exhausted GrowItem) the added elements would be ignored.