Update translation URL

"rosetta" now lives at translations.launchpad.net
Use standard "[y]/n" / "y/[n]" prompt

This removes the need for a translated prompt. Fixes issue #20.

Also,
* merge boolDefaultNo() and boolDefaultYes() into boolDefault()
* do not accept arbitrary answers, but prompt again
Merge pull request #39 from rfjakob/next

Update yes/no prompts. Fixes issue #20