make optional random-key vs. content-hash key

[zooko]

Recently on ​the tahoe-dev list there was a question about a Microsoft patent that might conver content-hash keys. (Short answer: not to worry -- Freenet had published the idea of content-hash keys before Microsoft applied for the patent. Also there are other reasons not to worry, such as that the patent seems to be about something else, judging from its abstract.)

But anyway, there are other reasons why someone might want to use a random key instead of a content-hash key -- random keys are faster and provide better confidentiality.

[zooko]

Fixed by 909d8480714fdbe0. To make this available in the wui and cli is ticket #294.