Adds bucketing to checks.

Adds bucketing to checks so that you can selectively run checks that may be expensive or logically grouped.

e.g.
   
    Stethoscope.check :some_check, :some_bucket, :another_bucket do |r|
       #ship
    end

Then to check only checks assigned to "some_bucket"

/heartbeat/some_bucket # check only the checks in some_bucket
/heartbeat # Check all checks