Make it clear if response was served from cache or not

@cohosh pointed out that it's currently not clear if a given bridge line was cached or not. This is problematic in an interactive scenario where a bridge operator is trying to fix her bridge, while repeatedly calling bridgestrap. Nobody wants to wait 24 hours until bridgestrap expires your cache entry. We should do the following:

• Make it clear in bridgestrap's JSON response if a given bridge line was cached or not.
• Consider allowing clients to bypass bridgestrap's cache, e.g. by adding a new field to the JSON request.